The Demonworld, splintered light and Magister Militum, and Alternative Armies Undead measure quite close to each other in size, closer to true 15mm foot to eyes (maybe 16mm). (measured one random pack of each) Cant find my undead BattleValor box, so cant be sure, but a lot of the BV depending on the race, are larger then some of the other manufacturers, but I cant remember BV undead being that much larger,and not likely to be much out of place (note: BV orcs and Hellian are quite large compared to other ranges (CLOSE TO 20mm)).
I dont have any Mirliton or Khurasan Undead, but they will likely match quite well to. Both of these are worth looking into also, as they tend to make quite nice miniatures
For what ever reason, size wise, most of the undead from the different manufactures tend to be quite similar in size. (guess undead dont warrant the scale creep:)
I havent painted any of my undead from any of the manufactures, so not sure which ones I like best,
Edit. found a couple BV endead skeletons, and they measure about 17mm, still a good match so not huge. Battle valor has some good undead "other" monsters worth looking into.
Splintered light has quite a variety of undead options, including egyption undead now.