Alright, so, here's the breakdown on scales:
The smaller you go the more realistic wargames look.
28mm is the bog standard GW crammed down your throat, but a standard game of 40k or WHFB or KoW (whatever your poison is) won't LOOK like a battle, it won't. You don't have enough stuff on a 6x4" table or 8x4" table for it to be a fight. If you do have enough stuff you don't have any maneuverability at all, you filled the table with 28mm doods, good job.
So the rule of thumb is bigger scales are better for skirmishing (although 6mm skirmish games are dope if you're in to napoleonics) where you have a handfull of dudes exploring terrain (or cheesecake board games like kingdom death). Generally things where you want individual models to look impressive.
As you drop down in scale a couple things happen: Painting becomes much faster, and the battles start looking more like a total war game.
For 15mm armour games (that's tanks chad) are much better, you get way more tanks and jeeps on a table, the dudes look great five to a base, and there's plenty of room for driving around your glorified hotwheels cars. Love it.
Fantasy in 15mm is this weird middleground between 28mm and the rest of the scales (coming after this). 15mm humans, or elves (whatever your gender is), still have a lot of great detail (especially if they're well sculpted) and are never over-designed (you know how GW likes to do it). So what you get are clean minis which look great at a distance, are easy to paint, and look great close up.
10mm Is the smaller slightly uglier cousin to 15mm.
6mm/3mm/2mm are all great but they function the same in terms of scale.
Once you get this small (2mm especially) you've lost a lot of detail (but checkout microwroldgames if you're interested in 6mm with lots of detail) but even with low detail models (irregular miniatures I'm looking at you) there's often a lot of great textures to paint and they look stunning when you've got an army of them.