Hey, I've got allot of experience in this realm, being an HVAC/Electrician not to mention installing high end residential generators and I sell my own brand of single phase transfer switches.
Just off your nameplate; your looking at 15A running, thats 14.1 RLA for the compressor and then .9 for your cond fan. RLA is running load amps, your minimum ampacity is 19, and the min fuse size is 25.
I would base it off that to start with, but you must also consider the indoor blower unit which is probably 120V and 3-7 Amps...for 2.5 ton my guess is 3.5-4.5 Amps.
So overall your looking at say 30 Amps. Volts x Amps = Watts. Outdoor unit is roughly 5200-6000 watts running, starting is usually rated at 125% of that. So now your talking at least 7500 watts just for the outdoor unit. The indoor unit will probably need at least 800, bot more like 1000-1200 watts just to be sure.
The most accurate way to do this is by measuring the starting amps with an accurate meter, that's how we do it for minimal use...but most of the time I've also sized for additional loads.
Like was said before, not all manufacturer's are the same, you need to look past residential generator units if you want a 8000K Watt generator to work for you. If you got a 10K or 12K you will probably be okay, start the indoor unit first though to ramp up the generator.
I like to use work duty rated generators like WINCO, Multiquip, MTQ... Onan, Generac, anything you would probably see at a construction site will be rock solid for AC units.