The basic principles to blowing up a miniature are essentially these: Build it as detailed as possible. If you can, replicate an existing building you can actually film in front of, because if you can file the real thing, at actual size, then cut to the explosion of the miniature, it will be much easier to 'sell' the effect of a full-size building exploding. Then, to add scale to the explosion, you have to slow down the footage. Traditionally, they over-crank the camera while filming the miniature, meaning they record at perhaps 100 fps rather than the standard 24 fps, so when it is played back at 24 fps it seems much bigger than it actually is. There are actually set ratios of speed increase based on the scale of the model. This is much more difficult when working with digital video, as virtually all cameras have the framerate locked in. This terrific tutorial by Tarn
can help, as it offers a technique to double your framerate, enabling you to slow the footage down much more without it getting choppy.
Building the model at the largest scale you can helps both with the detail work and the realism of the finished shot.