A 28mm figure would be a scale 7ft tall in 4mm scale, 20mm would make it 5ft but is probably the better size.
I have a set of the black tree daleks linked in a previous post, painted up as original daleks, 2 silver with blue spots, 1 red with black, 1 black with silver and a gold with black spots.
The only thing I need now is a suitable 'Doctor' figure, and then which one??
Whilst the daleks may be a bit big, most people don't seem to notice, added to which the real ones are big.
There is a photo 3/4 of the way down my layout thread on the previous rmweb:
http://www.rmweb.co.uk/forum/viewtopic.php?f=9&t=23685&start=25
Mike