It's good to remember that perfectly even blocks are a relatively new thing in quilting. You could sew this together as-is, taking off whatever extra you need to get it to fit, and it'd be perfectly appropriate for the time and style of those blocks. But if you still want to make it a little more balanced, there are a couple of methods.
Easiest is what CJ recommended- use borders to get all the blocks up to the same size. Pick a busy fabric with the same color background as the blocks, and the actual edges of the blocks will receded so you don't really notice if one's a little bigger than the other.
The other options were going with a non-grid setting like a brick pattern, purging anything that couldn't be squared up and using the excess blocks on the back, or cutting the blocks up into something else.
