A help page that hopefully answers a few common questions on wall railings
A common ask question is how high should I have the railings in relation to the top of the wall. This kind of depends on the type of brick pier cap you have. If a soldier course then the railing heads should be no higher than the top. but they obviously can be lower and its all down to personal choice.
The examples drawn here have a stone type cap. the first being the lowest, this is set under the pier cap. the middle set above the wall but under the top of the pier cap, and the final drawing with rail heads set level with the top of the pier cap.
The railing should have clearance from the wall, this allows air to get under the railing and stop the salt and the likes in the bricks from being in contact with the bottom rail.
The railing head choice has an effect on the overall size, some rail heads are shorter and some longer, but if you give us the overall size then the height of the ladder part of the railing can be adjusted in height to suit.