I hope my input will be worthy of your eager trust.
Blocks not being the same size: I quote from Victory Quilts by Elenor Burns - - if the blocks are within a quarter inch of each other, there is no cause for concern... okay, not a direct quote, but she said it in that book, and it worked for me. Of course, put the blocks in the middle as much as possible.
If you lay your blocks out, and sew the sashing JUST between the blocks, row by row, and sashing JUST between the rows, then a sashing around the whole thing, you will have saved yourself little pieces of sashing, there will be no wide sides and tops, they will be easier to keep straight across from each other ... hope that makes sense.
An example would be seven blocks across? Then, you would sew a sashing on the right sides of blocks 1,2,3,4,5,6 and nothing on the side of seven.
nine rows long? sew the long sashing on the bottom of row 1,2,3,4,5,6,7, and 8 ... join those rows, two by two, and then the twos together and then the fours together ... then the outer border/sashing ...
just follow one part of the instructions at a time, and it will make sense to you in the end ... <g> or, you can cut little pieces of paper and pretend/practice to see if that is a process you want to work with.