Its not just the item editor's tile previews that are too small. There's a whole bunch of em. Here's the rest of em I noticed:
All 3 of em in the enemy editor need to be 1px taller.
The ones in the shop type editor: 1px bigger in both directions.
Drop Set editor: 1px bigger in both directions.
Weapons/Misc sprite diealogs need to be 1px bigger in both directions.
Screen->Item dialog: 1px in both directions.
Catch All when room type = special item: 1px bigger in both directions.
Screen->Enemies enemy sprite preview: 1px bigger in both directions.
That should take care of all those things if anybody hasn't noticed any of these yet.