It's as simple as you putting in the bb you want and going out and using them irregardless of the results. Or it's as expensive and time consuming as you covering a lot of testing to fine tune and get the best result for your toy. Personally I don't run more than .25's in anything up to 350fps in full auto for my old AEG's (those without S-Hop). I've upped that to .3's through .4's for those with S-Hops as the result for distance is better, and run heavier in single shot bolt action or GBB's with double stack gas mags.
There's a multitude of factors that will change how your Airsoft toy performs, like the air seal of your particular toy through to cylinder volume and spring, or even the tolerances of your model versus another exactly the same etc etc etc... But here's one way to find a reasonable BB weight for you: (Apologies if it covers things you already know)
So firstly, knowing your own AEG and it's current setup will massively help, comparing it to the performance of other players toys of a similar nature will also aid in this, but do remember that not all toys are the same. Once you have a base line with your existing setup and have a good idea of the distance and flight level of your shots (ie how the hop affects the bb towards the ends of the shot), you'll be able to compare any changes you make from there.
Next is to remember that not all BB's are made the same - tolerances between BB's (even in the same packet) can vary across manufacturers. For example, you might decide to try those with better quality tolerances (Perfect Fit / Precision / Maruzen Super Grand Masters etc), they are likely to produce better consistent performance, but could also increase the FPS versus another version of the same BB weight (and cost you more). Also, if you buy those with a slightly larger diameter, you may find they jam in a super tight barrel or that your hop setup works differently with them. So if you test, remember to know what you are testing with and that simply keeping the weight but changing the label might result in different results.
Grab some of those BB's you want to try (I mean, fill a hi-cap or several low caps with .2's, .23, .25, or .28's etc) and note the manufacturer then go and test. Make sure you test in the same area you did your base line testing, same distance, similar weather (ie not rainy/windy) and if you can, in somewhat similar temperature (temp' changes may affect your hop rubber). I like to remove all the hop and slowly add a little bit at a time, helps to prevent you starting with too much and over hopping it sending the BB vertical lol. Remember, no hop might cause the bb to fall out the end of the barrel, so when I say remove all hop, know this and keep the toy level whilst you test and add a little hop until you can see it take affect then test from there.. On the flip side, do not add way too much hop or you will likely find it jams the bb inside the barrel or knocks your hop totally out of place (bad for s-hops). I also watch the shot from down the side of the barrel to get a better idea of the flatness of the trajectory and adjust the hop accordingly, looking straight down the sights / scope will not give you the best indication unless you're checking for hop / barrel twist sending the bb off to the side.
The results should hopefully give you the best indication.
Hope this helps, good luck in your choices!