Heated bed

Hey everyone I have a weird question about heated beds. My question is this do they need to extend to the end of your work area or can they cover like 80-90% of it? I’m going to try a make a riff (300x900) printer well more like a lemon breeze printer 500x900 to do some 1:4 scale modeling. But can only find 400 x 400 silicone mats. I was thinking of either using aluminum or glass as the bill plate. Any help would be great thank you everyone