Lack of standardized size suits them to file transfer/storage instead of use in portable devices
Physical details
Note that a memory card's dimensions are determined while holding the card with contact pins facing up. Horizontal measurement is width, vertical measurement is length, depth is thickness. For most cards length is larger than width, but it's not always true. Also, most cards include a direction arrow to aid insertion; such an arrow should also be facing "up".
The following chart gives details on availability of adapters to put a given card (horizontal) in a given slot or device (vertical). This table does not take into account protocol issues in communicating with the device.
Following labels are used:
+ (native) - a slot is native for such card.
D (Directly compatible) - a card may be used in such a slot directly, without any adapters. Best possible compatibility.
M (requires a Mechanical adapter) - such adapter is only a physical enclosure to fit one card sized into another; all electric pins are perfectly the same.
EM (requires an Electro-Mechanical adapter) - such adapter features both physical enclosure and pins re-routing as terminals are sufficiently different. No powered elements in such adapter exists, thus they're very cheap and easy to manufacture and may be supplied as a bonus for every such card.
E (requires an Electronic adapter enclosure) - these adapters are the most advanced ones with some chips (may be requiring external power) that transform signals, as well as physical enclosure and pin routing. Original card must be smaller in size than emulated one in slot, as original card have to fit inside an enclosure.
X (requires an eXternal adapter) - technically the same as E, but such adapter usually consists of 2 parts: a pseudo-card with pin routing and physical enclosure size that perfectly match the target slot and a break-out box (a card reader) that holds a real card. Such adapter is the least comfortable to use.
Empty cell - card can't be used in such slot, no single adapter is known to exist. Sometimes a chain of adapters can help (for example, miniSD→CF as miniSD→SD→CF)