I'm doing my rough designs for my pj and wondered what sort of size people are cutting down their fresnels to. Looking at the 320x400 mm fresnels on 3dlens it seems quite a fair bit could be chopped off.

Yes, there is a lot of lens that may be safely removed. You have to determine what precisely you require based on your component dimensions and proceed from there.

So is there a recommended size difference between fresnel and LCD? For example, should the fresnel be the same size as the lcd (plus mounting loss)?

In an unsplit design the light is already converging when it hits the LCD so the fresnels must actually be larger than the active area of the LCD plus any mounting loss.

So, active LCD area + mounting loss + amount light cone (or pyramid) diverges tracking backward through an unsplit design.

Hope that helps.

Damn, of course. Then in a split the light has been, er ordered so it travels straight through the LCD... I need to read more and ask less.

Thanks for the help!

You're very welcome. Others may follow in your path and read this preventing another question. Or maybe not.

Best of luck with your build. Someone's always around to help.