Jory, This is where a design wall is the best tool you will ever have!! (unless you are a math genius or whiz) Different size blocks can all go together as long as you have some fabric "bridges". Those pieces of fabric that will match up and even out the edges of your Main Blocks.

Think of it as a PUZZLE...get your blocks done and on the design wall..then figure out the size and color of "bridge" you will need to get it to match it's surrounding blocks. This is a time consuming method,, so don't rush the process... Granny