PCB enclosure tolerances
Just leave generous clearances and you should be fine. Even with well designed and made boxes (eg. Hammond) it's not unusual to allow a couple mm overall clearance (1mm all around). Eg. (from above datasheet- maximum recommended PCB size) 



Maybe you want to allow 1.5mm rather than 1mm if it looks a bit rough. Shrinkage (typically a couple of percent in linear dimensions, first-order compensated for in the mold design) in injection-molded parts is affected by resin choice and processing parameters so it's more likely to vary if the manufacturer is swapping resin types after the mold is designed or is pushing for high production rates. 

Keep in mind that there are always draft angles in molded parts (not always shown on drawings) so the inside will be smaller at the bottom (top of the mold core) than at the top. Otherwise the part would not come out of the mold easily (or at all). 

Similarly, allow generous hole sizes for mounting holes-  use at least the "loose fit" diameter in mechanical engineering sources\$^1\$. As well as linear shrinkage, bosses can bend a bit (the part is a bit soft coming out of the mold) if it is not handled perfectly. 

\$^1\$You can find that information online- look for "tap drill" tables, but the real bible, in North America anyway, is Machinery's Handbook.