MSX2's don't detect the amount of RAM afaik, they just initialize the mapper registers before starting the regular RAM search (BASIC / MSX-DOS 64K). Message under the boot logo is fixed. If you expand RAM in such a machine, or plug in external mapper, message stays the same. Not sure about VRAM size checking though...
Some only have message "VIDEO RAM: ...K", some have message "VIDEO RAM: ...K / USER RAM: ...K". But there exist some modified MSX2 roms out there that actually count the RAM. Not factory-standard, though.
MSX2+ and Turbo-R actually count the RAM on bootup. Multiple mappers, too. And (with a hardware reset), do so in a destructive manner, that is: erase some bytes while doing that count. Which in some cases can be regarded as a feature. 
- what is the minimum main RAM required?
MSX2: 64K. Can be a memory mapper. 128K is most common though.
MSX2+: 64K. Either must be a memory mapper, or implemented that way in any MSX2+ I know of.
Turbo-R: no published standard that I'm aware of. Lowest amount = 256K in the Panasonic ST.
- what is the minimum VRAM required?
I'd think 64K. But practically all MSX2's / 2+ / Turbo-R have 128K VRAM.
I must always keep in mind that WMSX is meant to be simple and compatible with the great majority of software, requiring minimum configuration.
In that case, go for 128K VRAM, and >=128K main RAM. 256/512 or more optional (or provide that as standard, 1 or 2 MB no problem Some software doesn't like full 4MB though).