There are two sizes of ExpressCard module: one is 34 mm wide (ExpressCard/34) and the other is 54 mm wide (ExpressCard/54). Both modules are 75 mm long and 5 mm high. Host systems can provide either a Universal (54 mm) slot or a 34 mm slot. A Universal slot, as the name implies, will accept either an ExpressCard/54 module or an ExpressCard/34 module. An ExpressCard/34 slot will only accept an ExpressCard/34 module unless an extender is used which would put the ExpressCard/54 module outside of the host system.

There is no direct performance difference between an ExpressCard/34 and an ExpressCard/54 module. In fact, both modules use the same 26 contact connector, so your computer is not even aware of which size of module has been inserted. Both sizes of module are able to take full advantage of the features of the PCI Express or USB 2.0 interfaces. However, the ExpressCard/54 form-factor does allow for greater thermal dissipation due to its larger surface area. This means that applications that run "hot", such as high-speed data transfer, may be designed into an ExpressCard/54 module to take advantage of the greater thermal dissipation.

Because performance does not vary with module size, module developers always prefer to fit their applications into the smaller ExpressCard/34 form factor when possible, but there are some applications that require the larger ExpressCard/54 module's extra width. Some examples of this are: SmartCard readers, CompactFlash readers, and hard drives with 1.8" platters. The media used in these applications is simply wider than can be accommodated by an ExpressCard/34 module. Other applications may be built in the ExpressCard/54 form-factor for the thermal reasons described above. Over time, as silicon technologies improve and novel approaches are discovered, applications currently found in the ExpressCard/54 form factor will migrate to the narrower ExpressCard/34 module. Applications in this category would include TV tuner cards and higher performance communication devices.

PC Card (often incorrectly called a "PCMCIA" card) is the general name for a technology originally developed in 1989 that adds expansion capabilities such as memory, mass storage, networking and wireless communications to computers and other communications and consumer electronics devices. Over 95% of notebooks use PC Card technology for their I/O expansion, although this number is already dropping due to the emergence of ExpressCard technology. CardBus is the name of the significantly faster 32-bit PC Card interface introduced in 1995. CardBus slots are backwards compatible with non-CardBus PC Cards, but a CardBus card cannot be used in a non-CardBus slot.