Finding right component package in Multisim
I don't know about Multisim, but in gEDA the footprints for these kind of components follow a pattern:

XXXnnn[n]{P}


Where XXX is the type of component, and nnn[n] is the size in mils, and {P} is polarized.

For example:


ACY400 is a generic 400-mil passive component (such as resistor).
ALF300 is a 300-mil diode.
RCY100P is a polarized electrolytic capacitor with 100-mil lead spacing.


(Axx = Axial, Rxx = Radial)

If there is a search facility I would start by narrowing the list down to show just those of the size you are looking for - such as entering "400" for a resistor - then browsing through.  You'll soon get to know what is what - of the thousands of footprints you'll regularly use a mere handful.