Here's my 510 in a Casemaster Super Bee dart case (thanks for the tip, rsngfrce!):

Top left: two new white cartridges in cellophane, two black atomizers with cartridges.
Bottom left: 4ml Johnson Creek glass sample bottle (about 2-1/2"H x 1/2"W). Two 3ml plastic bottles (about 1-7/8"H x 1/2"W) also fit. A 5ml PureSmoker plastic sample bottle (about 2-1/8"H x 7/8"W) will fit, but bulges the outside of the case slightly when closed.
Right: Assembled 510 with long manual battery and white cartridge, surrounded by two short manual batteries. Can fit three assembled 510's with long batteries. The bottom left pouch is too tight for a battery, but it doesn't really matter.
The closed case is about 5-1/4"H x 2-7/8"W x 1"D. A pack of 100mm analog cigarettes is 4"H x 2-1/8"W x 7/8"D.
I bought mine on eBay for around $10 with shipping since the stores near me didn't carry dart cases.