Your best bet is to try and get your 3d editing app's scale to match as close to 1:1 with dbp. I am having this same issue with my world editor (Valve Hammer3.4) and my 3d editor 3DStudio Max 3.1. What I did was make a room in VHE3.4 and in the middle place a 1x1x1(64 units) block on the floor. Then in 3dsmax a made a 1x1x1 block and loaded into dbp. The 3ds block was 5x bigger than the vhe block-sheesh! So I tweaked the home grid units in MAX down until 1x1x1 in max is the same size as 1x1x1 in vhe (ended up being Grid=1.87) Anyway, if you want to alter 3d objects created outside of dbp use:
SCALE OBJECT objnum,x%,y%,z%
so to shrink your object to half of its original size you'd say
SCALE OBJECT 1,50,50,50 (as an example)
good luck
-RUST-