What a great gift for the couple!!
Are you mixing in any other blocks?
If these are the only blocks, they all just need to be the same size-whatever size- so they will fit together. Then you can add borders to make it larger or to adjust it so it will match up with another set of blocks.
Here is one that I made:
http://www.quiltingboard.com/t-49147-1.htm
The siggy blocks are in the middle section and since there were not enough, I added borders and blocks to make a nice nap size.
And even though I drew out instructions on where to write....some people still wrote clear to the edge and didn't leave room for the seam. UGH! Make extra blocks just in case someone flubs up too! I had one teacher throw out 3 blocks and start over because she didn't like her handwriting.
It will be treasured whatever you decide!