pcontext:SetResolution(dpi)
This is a scale factor between points specified in a Pango font description and Cairo units. The default value is 96, meaning that a 10 point font will be 13 units high. (10 * 96. / 72. = 13.3).
Note that even though the resolution must be in "dots per inch", physical inches aren't actually involved. The terminology is conventional. A 0 or negative value means to use the resolution from the font map.