There aren't any default prints for that brick, so it's hard to tell. Letter prints are 256x256 with a 64x64 icon, but are stretched slightly. Make the print with a 5:6 ratio (this is the same as the brick's width to height), then resize it to 256x256 and see if it works.
When naming the folders, call it "Print_1x1_..." - prints for 1x1 bricks go on both the 1x1 and 4x4.