I found several models in GrabCad, that will suit my AutoCAD drawings, but these download 3D models are large and do not know how to modify them to my scale of the drawings.
One thing that I always take note before I start the drawing is the Units.
type units command. Maybe the 3D that you are going to incorporate from Grabcad has different unit of measurement than from your current project.
Let's make sure that the current unit is the desired one. Now regarding the scale. If ever you want to adjust It to your preference use the Scale, Reference command. Its very cool!
Good Luck :)
Also, when using scale, you can calculate the exact scale you need...
Since Metric and English don't always play nice for round numbers, always remember 1" = 25.4mm and 1mm = 0.039370078" ( 1/24.4... from my pocket calculator... you can probably get more digits from Moffsoft Calculator, a freebie. It also has a nice copy paste ability.)
If the Metric to English conversion doesn't solve it, measure the object you want to scale, and the dimension you need to fit it to. If it's too big, divide the small number by the big number... If it's too small visa-versa. Use the most resolution on your measurements possible... (1/256 if in English, .0.00000000 if in Decimal).