I'm sure it will one of two options.
1.) You build a component of the block (I would assume 16x16 - 32x32 -OR- 64x64) and then do like you would paste it, but it converts into a 1x1 area.
2.) You have to make a model of it from exterior sources (GIMP, PAINT.NET) and import your design.
I would assume that 32x32 would be the size used, since each custom block would probably have as much data as each block it's made from.
16x16 isn't nearly enough room to make something look good, while 64x64 is way too much data.
I've heard so many diffrrerent rumors about this, I'm just holding off to see it before I make tons of blocks. Same for NPCs.