Can anyone tell me what size sheets and bed for the fld120?

Go to a mattress store. TRY THEM OUT. You want good quality, with inner springs with a cooling memory foam topper. It wont be cheap, you will thank me later. If they dont have twin XL they will be able to get it.
Caviot Emptor... or something like that..
EDIT; AND...Buy new, good quality pillow or pillows. I cheaped out for years, cut up memory foam from Wallyworld and Costco, it's still in the upper bunk.... I dont sleep there.
I spent $80 on a charcoal memory foam pillow, it's to die for. The whole bill was almost $900. BEST $900 IVE EVER WRITTEN OFF.

 

Twin XL 80” x 39”

 

I make any comfy sheets work. Hospital corners do the trick for me.