Hmmm good question. I have done both, I have run in a local club 10k, my first race, and yes the bulk of the race ran off and left me no problem I plodded round and had a ball. I have run in a 10k with 10000 runners, where the first place runners were coming into the finish before my corall started and where I was triping over other runners for the first mile. I have also run in a half with 15000 runners where they got us all off in less than 5 min. I suppose a lot depends on how well the race is organised. Small races are better value for money. Big races have the wow factor.
Which do I prefer, well this September I had the choice of a half in Worcester, organised by Steve Cram, (British running legend) and which Paula Radcliffe (world running legend) has entered, so I suspect will be over 10000 runners, or the local Macclesfield half, which has a limit of 250. I have entered the Macclesfield race.

For a shorter distance race I like them small. Better chance of winning an AG medal and you don't have to dodge around so many people. Half or full marathon I like big. Lots of crowd support, big expo, good food etc.

I run a very large 5k and 10k each year. This is what my splits look like for 5k: 1- 1 min slower than I'd like to run. 2- 30 seconds slower than I'd like to run 3- exactly the pace i want to run. 4-30 seconds faster than I want to run. It takes me a full 2 miles to get around bodies and turn tight corners and I'm left with gas in the tank.

10,000 runners is good for a half though. That way I'm never running alone and don't start out too fast.

For running longer distances, I prefer medium-to-big races. I did local half marathon once and had maybe 500 people and it was so lonely. And there were hardly any spectators, and that was rough. But now that I'm marathon training and 13.1 isn't a HUGE distance for me, I'd be more willing to do a smaller race (looking at a different local half at the end of the summer that has 500-600 participants). I've done half marathons that had 10k participants, and that was a little nuts, but you were never alone and people were really supportive; I'd stop and walk and people would run by and encourage me to keep going (which I needed). Same reason I'm running a big running festival event for my 1st marathon: I'm definitely going to need the support and encouragement to keep my feet moving!

honestly it doesn't really matter to me for the most part. i've had a great experience at large races and much smaller ones.

i've run a disney half marathon with 20,000 people and the course was not designed to handle that many people. there were so many bottle neck spots that it would have been pretty frustrating except for the fact that we were running at disney world and knew ahead of time that we would not be trying to set a PR or anything so it was still a great time.

this summer i was also on the exact opposite end of the spectrum running 2 legs of an ultra relay and found myself running in the mountains in the middle of the night completely alone for about a half an hour before i caught up to another runner. just me and head lamp and probably a cougar or bear that i thankfully never ran into.

'crowd support' isn't something on my list of things i think about when signing up for a race. one race i don't think i'd ever do though is the vancouver sun run. 50,000 people on a 10km run? that's just way too crowded. unless you're out front there's no way you would be able to settle into a decent pace without constantly having to weave in and out of traffic.

I've been in everything from less than 30 people to upwards of 70,000. Trail races I definitely have a preference for small or even teeny tiny. There's a couple of very popular trail races here that can have almost 1000 people. I've done them a couple of times but I won't do them anymore; just too crowded.

Road races I guess it depends on what my goal is. If it's like a destination race and a PR isn't the primary motivation then something like the 40,000+ at New York is just part of the experience. If I've got a time goal in mind then I prefer small. I had a time goal going into Chicago and the masses of people just ended up frustrating me.

Interesting thread. I'm running my first half-marathon tomorrow morning. It's the Deseret News Classic. They've been doing a marathon since about 1970 but they only recently started having a half marathon. Last year there were over 900 in the half marathon and over 400 in the marathon which starts 30 minutes earlier and whose second part is essentially the same route as the half. So we're likely looking at well over 1000 runners in both of the races combined.

Last few miles is down the route of a parade which starts later in the morning so should be a good size crowd to encourage us as we get near the finish.

You're running a HM in the middle of the week?

Interesting thread. I'm running my first half-marathon tomorrow morning. It's the Deseret News Classic. They've been doing a marathon since about 1970 but they only recently started having a half marathon. Last year there were over 900 in the half marathon and over 400 in the marathon which starts 30 minutes earlier and whose second part is essentially the same route as the half. So we're likely looking at well over 1000 runners in both of the races combined.

Last few miles is down the route of a parade which starts later in the morning so should be a good size crowd to encourage us as we get near the finish.

You're running a HM in the middle of the week?

Here in Salt Lake City, July 24 is Pioneer Day and they always have events including a parade, races (marathon, half, 10K, and 5K), fireworks at night, etc, on whatever the day of the week it falls on (unless it's a Sunday in which case it's moved to Monday).

Curiously a few months ago when I was poking around various websites to generate a training plan and trying to enter tomorrow's date as the race day, at least one of them refused to acknowledge a Thursday as being legitimate (wanted it to be on a weekend).

I ran a marathon with 70 people this past weekend. Race before that was an ultra with 40 finishers. Love the tiny races. Always a better chance of winning lol. Plus I like to run alone & that's what I do in these races :-)

Oh, I do like the smaller races for the possibility of placing. I've been able to win in my age group a few times in a few distances. :-)