I find 3'x3' to be just fine for pretty much anything I want to run, but I'm a little persnickety about my scale-measuring.
Essentially, use a multiple of the individual figure scale for in-game scale inches. Figures are 28mm? One inch equals (roughly) one inch, 28mm. Playing in 15mm? One game-inch equals 15mm, or about half an inch--1.5cm. Playing 10mm? One game-inch is a centimeter (and you're going to have room for tonnes of maneuver).
For me, the reason is pretty straightforward. When my figures scale down what I'm really interested in is obviously maneuver, because what I really want to do is see more board. If you don't change the scale of the movement unit you don't really gain any terrain. Movement remains the same scale; it takes no more or less time to travel from one side of the board to the other. If you link movement scale to figure scale, by scaling down the miniatures you're effectively zooming out on the map.
That's a huge win in my book.
(There is one reasonable argument for not scaling movement for a smaller scale miniature, and that is because for whatever reason you find it very difficult to reconcile the size of a miniature intended to represent a 1:1 scale element with the implied speed of a turn in your given game. Because THW rounds are not strictly defined in terms of actual time measurement, it's not a game that I think of exemplifying that particular problem – but it could be an issue for some people. For them, leave the movement scale alone and just use bigger maps. For the rest of us? Scale away.)