My first outing from 4 strings i went the full hog, missed out 5ers and bought a cheap ESP Ltd 6 stringer with 16.5mm spacing measured at bridge. No idea what the nut spacing was but i think it was narrower than my Jazz or Thumb bass which both have narrow nuts compared to P basses in general. All my other basses were circa 18mm at that time. I know that doesn't sound much but i found i was missing the strings with my plucking hand. I did eventually get used to it but when i had my Overwater 6 string built i requested 18mm spacing and it made a difference in my playing. My left hand managed to adapt far quicker at changes in string spacing.
My 5 stringers since that first 6er have generally been around the 18-19mm. I found a larger spacing was easier to get used to but then you have the issue with the neck being slightly wider.
I'd measure what you have and go for a compromise. If you have 18-19mm at bridge and don't want a wide neck then maybe stick with something 18mm or less. If, like me, neck width doesn't bother you as much then stick to what you are currently used to.
No matter what you buy you stick at it and you'll get used to it.
One other point i should mention that when i had the ESP with 16.5mm spacing i had no issues changing back to my 4 string basses. It was always when i went back to the smaller spacing for some reason.
Hope this helps
Dave