What Tire Size?

If your bike is stock, the 100 rear should be fine. If you've done the usual mods (pipe, jetting), then the 110 is the ticket. There's only 1 size for the front. Just recently installed front and rear (110) 773s on a friends KDX200 - huge improvement over the stock tires!

Be aware that the 110 will change the bike's attitude a bit, so you should check / reset the rear suspension sag.
btw - if you' have short legs, you might want to use the 100 rear regardless of engine mods.

I like the MSR heavy duty tubes from Rocky Mtn. for off road use. They are however,heavier than the thin tubes and thus add weight as does bigger tires. The additional weight is a small penalty to pay for the reliability and performance improvements of big rear tires and HD tubes. I guess all this is worth what it cost you--lol. ride on John

Vaulta, your front wheel is 21" - there is only 1 21" 773, a 80/100x21. Your rear wheel is 18". You have 2 choices here - a 100/100x18 and a 110/100x18.

Yes, I would recommend 110 based on your feedback. Run about 13 psi in the front and 12 psi in the rear.

Heavy duty tubes are recommended. The stock tubes should be tossed out with your old tires. John's MSR recommendation is fine.

Tire size primer - the 1st # is the width, the 2nd # is the sidewall height (expressed as a percentage of the width), and the 3rd # is the diameter.
As both the 100 and the 110 have a 100(%) sidewall height, the 110 will be 20mm (2 x 10mm) taller than the 100. This is why a shorter rider might want to choose a 100. The 110 tire will jack up the back end of your KDX, transferring more weight to the front and possibly changing the rear suspension sag.

What part of FL are you in, and where do you ride?