I have acquired a new, un-assembled CY Models P-51 with 96" wingspan and bought a DLE 85 engine. Can anyone tell me which prop size would be best for this setup?

26/10

26x10 is a Pretty good prop for normal Ga models on the 85 , But you may want more pitch for speed, so something like a 25x12

Most of the Texas Warbird Thunder pilots are using APC “T6” props
(20x15) for up to 60cc engines.
Going on up to 75-80 cc.engines we slip the APC F-1 GT prop which is
(20x18).
The trailing edges of these props are “cupped” and really make a 85-88” warbird come alive. I’d say on a DA or DLE 85 a 22” diameter x 22” pitch
APC will give you the “pop” you’d be looking for. I used a 26x10 on my DLE 85 and found the 22x22..... You’ll be pleasantly surprised.
WT