Thanks for your answer, but my problem is exactly to know how to scale the objects. if you scale a "a" to match with a "l", your "a" will be too big.

Hello! I have the following problem. Let us assume I put an equation into my Inkscape document ( Extensions->Render->LaTeX...). Probably I will scale it. Later, I want to insert another formula. What is the trick to scale it, in order that the scaling is coherent with the one of the previous formula...