Here is a list of known issues of Mapserver affecting Cartowev.
Mapserver 4.8.1 & CartoWeb (Any Versions)
Size of SYMBOL of Type PIXMAP Can Not Be Set
In the latest version of Mapserver, one can not set the size of a symbol of type PIXMAP (gif, png, ...)
see http://mapserver.gis.umn.edu/bugs/show_bug.cgi?id=1684
The CVS version of Mapserver has been partially fixed, as you can only controls the largest pixmap dimension, either the height or the width.
Partial solution: compile Mapserver from the CVS.