A lot of has to do with history. Unlike the United Methodist Church which is undergoing a major split at this time, the Presbyterians have taken the route of smaller spin-off denominations. As the mainstream united church (currently the PCUSA which was formed from the northern and southern branches in the 1960's) has drifted toward greater liberalism, conservative members have departed, leaving the more liberal members in the denomination. Some of the conservative folks have formed conservative Presbyterian bodies such as the Bible Presbyterians, the OPC, the PCA, the EPC, etc. Each was formed as a result of a difference on a particular doctrine or practice. For example, homosexual ordination was the trigger for the EPC, but played no role with earlier spinoffs such as the PCA and the OPC. Women's ordination was a huge issue for the OPC, but not so much for later spin-offs. If one opposes women's ordination either as elders or as deaconesses, then the OPC would be an excellent choice. However, most Presbyterians today do not perceive that as being a significant issue. Thus, they have other options from which to choose. As a result, the OPC remains quite small, but hardly the smallest in the family. In the USA the RPCNA is probably the smallest. Their issue goes all the way back to seventeenth-century Scotland and revolves around taking the Covenant. So, if the Covenant is the big issue for you then I highly recommend the RPCNA.
Ancillary issues are there, as well. One of the more interesting ones IMO is the singing of the Psalms, which is fast fading even in the highly conservative branches, other than the RPCNA. So, if exclusive non-instrumental Psalmody is your issue, then go for the RPCNA.