Luckily for FM Towns it doesn't use prioprietary-ish memory unlike X68000
Mr. Labo's website says the stock RAM value for computers. This is the embedded RAM. All computers except Marty can have RAM expanded by using 72 pin RAM sticks that are non-parity. Referring to Mr. Labo's page it says the maximum RAM that can be used in each RAM slot. 60ns to 100ns timing is acceptable. I would recommend 60-70ns if you can get it. It shouldn't be too expensive. Don't use RAM that's faster.
The bottom line up front is that you don't need to “maximize' RAM in an FM Towns computer. If you add too much you'll actually bog down and prevent software from loading quickly. Try loading Indian Jones with 26MB of RAM in a tower model. Then try it with 8MB RAM.
Arguable the ideal amount of RAM to have in a Towns computer is no more than 10MB. I would argue that from all the retail software there aren't any that require more than 8MB of RAM. At least none that I've dumped.
When FM Towns boots up it will perform a RAM check and depending on your jumpers on your RAM stick (if required) it will detect a certain amount to be used or not.
Pay attention to the red rectangles. These are jumpers you must configure. The RAM sticks in the photo are 8MB each. If you want to achieve a certain amount of RAM in an FMT you must solder the jumpers in specific configuration.