The X protocol (and server) come with a pre-defined set of cursors. If your program is happy with one of these, you use it by telling the X server that you want cursor number N with XCreateFontCursor(). As mentioned in the manpage (and hinted at by the function name), the server loads these cursors from a specific X font, which is exposed to clients under the special font name ‘cursor’. Like the special ‘fixed’ font name, this isn’t even a XLFD font name and so there’s no way to specify what pixel size you want your cursors to be in; you get whatever (font) size the font is or the server decides on (if the X font the server is using is one where it can do that, and I’m not sure that the X server even supports resizable fonts for the special cursor font).