Size ( in pixels, or model units ) of a string

I am drawing text in my Compute() method using:

Prs3d_Text::Draw(aPresentation,aTextAspect,textCollection,ptLocation);

In order to position the text and the surrounding graphics, I need to know the size that the string will be when displayed. For now I'm using Graphic3d_NOF_ASCII_MONO, so there should be a fixed ( per font height ) character width that I could multiply by the number of characters. Does anyone know how to find the character width?

Also, in the future I'd like to support proportional fonts, so if there is a way to get the size of a string in a proportional font, I'd like to hear about that too.

Thanks!