what are the dimesions of the large size unit

39"Width  39"Length 19" Hight at the front 27" Hight at the peak

can you get it in a 3ft.x 12 ft. size?

You can order 3 or 4 and  they will link together for your desired size

Is there a base to the Cold Frame, or does it just comprise four sides and a top?

The Coldframe is only the sides and the top there is the option of placing it on a raised bed that is open on the bottom.

How easily does this item assemble and disassemble and could it be done seasonally

Yes the panels can be taken apart every year,or you can remove only the top.