There is no easy way to design 'same size' blocks using different patterns inside the block. Most designs knit to a different 'gauge'. Therefore, even if you cast on the same number of stitches for each different block, each block can turn out to be a different size due to 'gauge'.
There are some dishcloth leaflets
(for sale at places like Joann's
) that have maybe a dozen dishcloth patterns that will usually come out the same size if you use the same yarn and needle.
That's what I would do for a blanket. If the leaflet has a dozen dishcloth patterns that state they come out the same size, just knit about 3 or 4 of each, and then stitch them together in random sequencing so the same dishcloth isn't next to her twin!
That said, I speak from experience. Several years ago, I made each of my granddaughters blocks blankets/patchwork quilts, using pattern stitches I found in a KNITTING DICTIONARY. What a headache getting them to conform to the same size! Well, they didn't...as hard as I tried. It ended up being as much work for blocking and seaming as the knitting itself. Oy!
Even though I spent hours blocking them to the 'same size'...the blanket was still kinda catty-wampus!