Ideally, you would want to have all the block sizes easily divisible by the same number so they would fit. You might want to consider padding them out so they all finish at a multiple of 6" So you would pad the center panel out to 24.5 X 36.5 (trim 1/4" from the long side). Small blocks: make them square at 6.5 X 6.5 medium blocks 12.5 X 12.5, and the long panels 6.5 X 12.5. I would use a neutral color for the padding, white, cream, grey, black...
I don't know of a site that you could use. I would go with graph paper.