It's not likely that "customization" includes memory allocations. The wise folks at ST have already anticipated the need for different memory sizes, and offer different versions with different SRAM and Flash size.
Go to a distributor and order the part number that has the memory allocation you need. You can obtain the relevant part number from the "product selector" tab on the STM product page for the H7.
I would expect those part numbers to be in the data sheet as well. Look for the part number matrix.
You didn't say which H7, either... there are quite a few variations.