SSDs themselves do write combining. They cache writes in the onboard cache until they have a big block and then they write the block in parallel to many NAND dies. This is how SSDs achieve such high write speed despite the fact that the write speed on the NAND itself is quite low. As long as the writes are more or less sequential the size is not very important (not until the number of write requests saturates the ability of the controller to handle).
Depending on the rate at which you need to write the strings, it may make sense to gather them up into blocks at least as big as the page size of the NAND in the SSD. Nowadays the page size is usually 8KB.