The only kind of memory card supported by commercial PS1 software is the SCPH-1020 (and sufficiently accurate clones).
Its capacity is 1 megabit (128 kB) divided into 16 blocks, of which one reserved for the filesystem, hence their nominal capacity of 15 blocks. No standard unit is ever used in official marketing material; only 3rd party clones have prominent text to the effect of "1 Mega".
The SCPH-1020 looks like so:
--------------------- | | | | | [PS logo] | | PlayStation | | | | MEMORY CARD | | | | | | |-----------------| | | | | | | | | | | | [label area] | | | | | | | | | | | |_________________| | | SONY | ---------------------
Other sizes are plainly not possible by design and imply a bankswitched (multi-partition) card or false advertising, whether of a PS2 card or outright flash fraud (especially with claims of "compression"); of course, "multiple of the above" may well be the answer!
The simplest bankswitched cards, typically 2 or 4 megabits, have a physical button or switch; others, especially ones with a numerical display on them, require button combos from the controller (typically [Start+]Select+L1/R1; other combinations may be implemented including an "erase all" one).
Official memory cards use 3.5 V power only due to their 3.3 V logic; this is not a given with third party ones (regardless of their reliability or official price) which may result in incompatibilities with a SCPH-75xxx or newer (unless trivially modified to restore the 7.5 V line).