Dimensions for EV3pieces
I don't know of any comprehensive parts dimension reference, but following should help you get started.
You can calculate part dimensions using fundamental dimensions of LEGO parts. The distance between holes in technic bricks is 8 mm (8 mm is one of core dimensions in whole LEGO).

So for example, lets take a beam (aka liftarm) with 7 holes, then length is 8mm*7=56 mm, and width is 8 mm.

Same rule for pegs, e.g. black pin with friction (used to connect 2 beams) has length of 8mm*2 = 16mm, diameter of 4.8mm - 6.2mm (depending on what part of pin you would measure). 

LEGO technic axels follow same rule: their length is a multiple of 8.

You can read more on LEGO dimensions at  http://www.robertcailliau.eu/Lego/Dimensions/zMeasurements-en.xhtml