when i try to assemble any part in it seems to have larger dimension than the main body ,i don't know how to overcome this...
You will need this when you have used a downloaded model with one you are currently modeling.
First measure the size you want your model to be. and also its original size.
For eg. I downloaded a tyre of a car which is 200mm dia. I want to add it to my car, but My planned dia is 75mm
You have to open the original part first. Select the scale tool. Enter the scale value as 75/200 (Wanted value/ original value) .
Save and go back to assembly to find the resized component.