Welcome!
Not quite sure where you are measuring on the downstem - it's a tapered plug and you don't clarify where you measure 14mm.
Take your measurement at the widest part of the ground glass joint - that is the measurement (roughly) of your opening. 18, 18.8, and 19 all mean the same thing.
If the widest part of the tapered male end is around 14mm, there you go.
All measurements are based on the widest part of the male joint. The female end needs to be a smidge bigger obviously, and it depends on where you measure, and the type of female connection - some have a recessed opening, if that's the case, you might measure up to 21 or 22mm
It's tough to tell what kind of setup you have - is it a standard straight tube with a downstem which has a female joint on it? Maybe you have an 18mm downstem and the female end of the downstem is a 14mm?