Comment 2 for bug 1422128

Reproduced on Windows XP, Inkscape trunk rev. 13979.

Sans was a generic font name used in 0.48, but changed in 0.91 to sans-serif in order to be consistent with CSS generic font names (see http://www.w3.org/TR/CSS21/fonts.html#generic-font-families). Replacing Sans with sans-serif in all affected text should fix the "Font not found on system: Sans" warning.

Note that generic fonts are a fallback mechanism, and that Inkscape maps the font to a real font. The size issue is due to the fact that the generic sans-serif font was mapped to Arial (used in the bitmap copy at the bottom of your file) in 0.48, and is now mapped to DejaVu Sans in 0.91, hence the size difference.
Weirdly, the font preview (in the Text and font dialog) shows an Arial font when the invalid Sans font is selected...

Thus if you need your file to look exactly as it looked in 0.48, you can replace your Sans fonts with Arial as a workaround.