How do you determine how many servings and serving size?

Don't overthink it, simply search for 'taco salad' and select one that seems right to you.

There are probably a lot of different ways you could decide to use to figure out how many servings or it's size you would get. ... It does, as mentioned above, depend on your calorie goals and eating habits and using the total calories of the dish as a way to determine how much of it you could have ...

I can think of 2 other ways that I have used in my meal preps that might give you ideas.

One is deep into calculation based on total servings of a macro in the salad, like protein, based on the suggested servings sizes on all the labels from those foods ... and then dividing that total into a reasonable portion ... For example, if you used 1 pound of beef (4 full servings) a 15 ounce can of refried beans (3.5 full servings) and 8 ounces of shredded cheese (5 full servings) .. that would be a total of about 12.5 servings of protein ... so the salad could serve that many portions You could use that as a basis for how many meals that salad would make, or double the number for how many sides. (I'm purposefully ignoring the vegetables, salsa, or sour cream you might be using for this example, but they would eventually need to be counted in for calorie or macro counting.)

Another way, that many have used is to weigh your container empty and mark that down. Make your salad and weigh your full container again. Then subtract the weight of the container to see how many ounces or grams (whichever you used) the total dish makes. Then, in MFP you could say one of those measurements is a serving. Then .. when you serve it up, weigh out how many units of measure you put on your plate and let MFP tell you how many servings of that size you could get from the whole thing. ... by dividing the total weight of your salad by how much you had put on your plate that you decided was a serving. (this method requires you to create a recipe on MFP - My FittnessPal)

No easy way around it ... it'll take some calculating and tooling around no matter which way you go.

PS .. I have used the second method I described for my soups/chowders/stews because then I can just weigh my plate, tare it out to zero, and fill it up with the food and enter the weight as to how many 'servings' I ate of that recipe with 1 oz or gram (whichever I used) being one serving .. so if I plated 8 oz or grams (whichever I used) then it would be 8 servings into My Food Log.

Nothing in OP's post suggested she was in 'analysis paralysis' or ready to give up.
We don't need perfection, sure, and not everyone needs to be very precise in their logging. But too much inaccurate logging and next time OP might be asking 'why am I not losing weight'...

PS: logging a recipe like a taco salad hardly seems like 'too hard'. I'll admit to sometimes avoiding dishes where it's hard to know how much I consumed (for example stews where a lot of the total weight of the dish is liquid that isn't consumed) when I'm cooking myself. Logging most regular meals isn't that hard though. If your way works for you, great, but we see a lot of people asking why they're not losing weight and logging inaccuracy is a possible reason.