A "light jet" is often a Citation 550 or a Beechjet.

A "medium jet" is a Hawker 800 or Citation Excel.

A "super-midsize jet" is a Hawker Horizon or Citation Sovereign.

A "large jet" gets you into Gulfstream/Legacy/F900/BBJ range, if I had to guess.

In the airline world, any 'RJ' is considered a "small jet". Small compared to other airliners yes, small compared to bizjets not at all.

There aren't many light jets outside of VLJs that are under 12.5; the most notable exception off the top of my head is the Cessna CJ series.