See discussion over at WordReference http://forum.wordreference.com/threads/dimensionner-une-pièc...

To quote from that webpage (in case it disappears), one contributor says:
``dimensionner = par exemple calculer sa résistance de la pièce quand on lui applique des efforts. En clair il ne s`agit pas de faire la cotation de ces pièces mais bien de déterminer par des calculs quels dimensions et quelles formes celle-ci doit avoir pour remplir correctement sa fonction selon le cahier des charges.``

To which another responds:
``To me, dimensioning is entering those calculated dimensions onto an engineering drawing, rather than calculating them in the first place. Sizing is an informal term, and doesn't necessarily apply to all kinds of dimensionnement. It seems to me that we`re more likely to use the general term design/designing,or perhaps the combination design dimensioning.``

If those two contributors are right, dimensionner and to dimension are more in the nature of faux amis.