As you're obviously aware, dimensions of taps vary very much from manufacturer to manufacturer and from style to style, but websites these days are usually pretty good at providing technical diagrams with dimensions detailed. www.bristan.com www.franke.co.uk The people at www.tapstore.com are also very helpful.
This document on Axis taps gives a pretty good step-by-step guide (if this is the focus of your query, which I'm not sure about): www.aqualisa.co.uk
Wall feed bath taps
Wed 9th Jan 2008
SVS
I was looking for some data on the fitting of "side valve" taps feeding an overflow/waste unit - such as shortest depth of tap available - side entry and side exit feeds if available and any help with manufacturers Ideal/Bristan etc