One cubic foot is 12"x12"x12", or 1728 cubic inches.
To figure out the dimensions you need, first find out how many cubic inches in your box:
1.2 cu ft = 1728*1.2 cu in = 2074 cu in
Now, pick a dimension that you want. Let's say you need a box that's 8" high. We now divide by that:
2074 cu in / 8 in = 259 sq in
Now let's pick one more dimension. Let's say that we need it at least 11" long so we can fit our 10" woofer in it. We'll divide again:
259 sq in / 11 in = 23.5 in
So our unknown dimension is 23.5". And to verify, 23.5x11x8 = 2068 cu in, very close (within 1%).