Theoretically there are no limits to megaROMs, but of course, there ARE limits for the currently known megaROMs. At the moment, 'cause they use a 1 byte (8 bit) selector, they could manage up to 256 pages. As you said, 256 x 8 KB = 2 Mbit and 256 x 16 KB = 4 Mbit, but it is not as easy as it seems, 'cause Konami mappers, for example, do have a limit and only use the lower bits of the selector and reserve the others for different purposes (SCC, etc.).

asMSX has its own MegaROM support and therefore all the examples use the dedicated macros. But if you just disassemble them, you will see how it words. Basically, you just use a memory write to a given address (or range of addresses) and then the bank is switched. As easy as that. But you need to know how to order your code in different pages and make sure that you always load the right one.