I know what full length handguards look like, but what's the difference in length between standard carbine, and mid-length?

I see them for sale around the web in differing sizes, but no lengths are given for them.

JoeLad

If I remember correctly midlength are 2 inches longer than the standard carbine.

From armalite website:

3.  Shift the gas port.  ArmaLite has shifted its carbines gas port two inches forward compared to the M4 Carbine.  Since the commercial minimum barrel length is set by law, this change serves reduce the gas pressure, delay its transmission to the carrier group, and reduces the length of barrel (and thus gas pulse duration) beyond the gas port.  This change is easy in a commercial setting but has logistics implications for the military.

So would the longer mid-length handguard allow one to use a bayonet on a 16" preban AR?

yep!

Sort of.  The ring on the bayonet won't be at the back of the FS like it is on the 14.5" with CAR HGs or 20" with full-length HGs, but it will be on the FS unlike a bayonet on a regular 16" with CAR HGs.