The minis themselves vary. Some of them are very close to your standard Medium D&D mini, some are Large, some are a slightly terrible size between Medium and Large.
You can pop the clicky outer/lower part of the base off some of the Mage Knight minis, and the inner base is about 1.25" in diameter. Very close to acceptable, but too big by a quarter-inch!
The Heroclix bases and some of the Mage Knight bases are of a slightly different design than the above mentioned bases. These minis are attached to the bigger portion of the base, which makes it a pain in the neck even to do the eponymous clicking thing, let alone pry part of the base off, and even if you did get it apart, you'd still be stuck with the wider base.
The Dreamblade minis are all on 1.5"x1.5" squares, which is even worse. If they were 2"x2", that would be fine for Large, but this in-between nonsense is unacceptable.
So, my aesthetic sense refuses to allow them to be used too regularly as-is. So, a rebasing quest!
I'm a little leery of destroying the monetary value of these things in order to turn them into things that are more valuable by dint of being more useful to me, but they were pretty close to dirt cheap, and they're not as likely to appreciate in value as genuine 3.5e minis are. (Though the D&D Minis line has been discontinued, D&D will always exist, and even
Although (unsurprisingly, given their limited-edition nature) the CoH/CoV heroclix currently seem to be selling for more than your average D&D mini on eBay - maybe I shouldn't have taken them out of their original packaging. Oops! I could have made back all the money I spent on that CoV box (on which I already effectively made a $10 profit, because I paid $5 and it came with a free month of game time worth $15, plus I still get to keep the in-game badges and other goodies). Oh well.
The Heroclix and Mage Knight figures are simply glued onto their bases, and are easy to snap off, with a knife or sometimes just by hand. The Dreamblade minis seem to be sculpted to their bases, and will need to be cut free with a very sharp knife.
The first step is to find new bases to which to rebase them. And the first step there is figuring out exactly how big the bases need to be.
Well, I figured it out, by measuring and then confirming on the internet: Medium minis use a 25mm base, though 1" would be close enough for government work. Large minis appear to use a base that is very close to 2". If you happen to find a mini small enough to qualify as Small, they use a base almost exactly the size of two US pennies stacked on top of one another (not that I would ever advocate defacing the currency of any country, that's a federal crime), or 20mm. If you find a mini large enough to qualify as Huge, that base is actually 3" (the smaller ones are presumably just shy of 1" and 2" to fit better in close proximity to one another, while you're not likely to have multiple Huges, there's enough space in the corners to have some maneuvering room anyway).
All mini bases appear to be 3/16" thick, but I'm guessing one can choose 1/8" or 1/4" without too much noticeable disparity.
I've got one mini that definitely qualifies as Large, and the rest are pretty firmly within the upper-Medium range. This is going to be irritating, I'm probably not going to be able to get one 45mm base (or even just a small number of 45mm bases) and a bunch of 25mm bases.
The next step: acquiring new bases of the appropriate size.
We could just use US quarters. They're very nearly the right size, and you're likely to have them on hand. However, they cost you $0.50/base (or $0.25, if you're okay with bases half as tall as all your other minis, which might actually help slightly to ameliorate the problems inherent with the non-D&D minis being larger in scale than D&D minis), approximately the worst available value. Plus, as above, the federal crime thing. Also, quarters have ridged edges. So don't do this.
Gale Force 9 offers Econo Bases, with 125ish or 250ish bases for $10 or $15. The smaller amount winds up approaching $20 when you include postage, for about $0.16/base. (These guys make you create an account before they'll tell you how much shipping is, which makes me want to punch them and also withhold my dollars from them.)
LITKO Game Accessories offers Circular Miniature Bases. I probably won't need more than 50, because I only have about 42 minis that need rebasing and I don't intend to acquire too many more. (These guys let you get a number for shipping costs without making you make an account or fill in any information, which is nice.) This winds up being about $15, or $0.30/base. (If I wanted 100, it'd be $18, for $0.18/base, but that extra $3 is a frivolity.)
Warhammer and Warhammer 40k bases ("bits"? "bitz"?) seem to be going for much cheaper from various sources, but I can't seem to pin down an actual detailed description of what these are like. From what I can tell, their edges seem to be tapered in a way that D&D mini bases are not (a trivial concern, yes).
You could, of course, go to the nearest craft store and see if they sell small wooden circles of the appropriate size. But as long as we're rebasing because a circle (or square) is a quarter- or half-inch too big, we might as well get the new bases perfect, and good luck finding 25mm x 3mm circles at a craft store! They mostly seem to sell them by the inch, which might be close enough.
Read the thrilling conclusion to this epic tale here!