Hi Alan
usualy in Creo you draw the sketch and then dimension to suit, it does come up with some interesting default dimensions 
but when you get used to it it seems pretty quick to just delete and redimension.
hope this helps
Jeremy
Alan,
A great strength of Creo 3.0 is the way it helps you create and manage robust sketch geometry.
All the time you are creating geometry, it looks for, and applies geometric constraints like parallel, perpendicular, equal lengths, etc.
Once you stop drawing geometry, and activate the 'Select' arrow, Creo displays the geometric constraints and applies 'weak' dimensions coloured light blue. Creo will not let you have an under or over-constrained sketch.
You can add your own dimensions which are a darker blue colour and Creo will remove and re-arrange the remaining, weak dimensions, If you drag geometry, the dimensions will change but you can prevent this by selecting a dimension then right clicking and from the floating menu click 'Lock'. Locked dimensions are brown in colour and will not allow you to change this value by dragging geometry. You can change a locked dimension by double clicking and typing in a new value.
If you create too many geometric/dimension constraints, Creo will open a Resolve Sketch dialog listing the constraints that clash, letting you decide which ones to remove.
I like to lock all dimensions creating a 'fully constrained' sketch so that nothing can change accidentally.
Project creates new referenced geometry coincident with the underlying geometry.
Creates new referenced geometry at a distance you specify.
