The typical door has 1/8" clearance on each side and 7/16" at the bottom. The door is 3/8" less than the opening width. However, many hinges with Full Backing Plates require 1/4" at least on the hinge side. It is better to get the door a little small than too big.
When you order a door from us, we need to get the opening sizes and how out-of-square the walls are. We'll make the deductions and we'll make a drawing and send that back to you for double checking. Once you say it is okay, then we'll order the glass and in about two weeks it will be ready.
Measure the width at top, the middle and the bottom. So long as the middle is not smaller than the top or bottom you can forget about it. The very bottom of the tile wall often curves in. Watch out for that. It is hard to measure accurately to inside corners because the tape has to bend. One trick is to make a mark on the tile and measure each direction from the mark, then add up the two dimensions. At the top, you can use a ruler or framing square. Hold the framing square in the opening and measure from the end of the framing square to the other side, add that together. This will give you a more accurate measurement than trying to bend the tape measure into the corner or holding the body of the tape in the corner.
The height can be anything you want; whatever looks good to you. 70" or 72" is common and I generally subtract the 7/16" clearance from that dimension so that the top lines up with the requested height. If the difference in top and bottom is 1/16" I would consider making the door fit the smaller dimension. It is up too you.
If the difference is 1/8" or more, I would make the door out of square so that the sides line up with the tile. A long level will tell you how out-of-plumb your walls are as well as whether they are bowed in or out. If a wall is bowed in then you must measure from the middle. The glass has to be cut straight.
Measuring Complex Shower Doors (NEO ANGLE):
Neo Angles are actually pretty easy to measure and install. If there is a knee wall and it is plumb and square things are still not too bad. If the panels go to a drooping ceiling and there are two out of square knee walls and the sill is cock-eyed and the walls out of plumb, then we have software for that. Just write it all down and we'll figure out the glass sizes.
At points B and C I marked the centerline of the curb by measuring the width of the curb and basically marking the centerline - of the curb. Where any two lines intersect that tells you where to measure. So you get three measurement: AB, BC, CD.
Then you take your expensive six foot level and measure how far out of plumb the walls are. (A four foot level is just not long enough.) Write it all down and send it off to NYC Shower Doors by fax, email, or phone call.
Your out of plumb measurements should be accurate to 1/16". The centerline measurements are not really even that critical because you can move the glass in and out on the curb to get it to line up as you install.
The height of the Neo-Angle should be whatever you want it to be, however you might think about having the top edge line up more or less with the height of a nearby door or with a grout line.
People often want the Neo-Angle to be a little higher than normal because you have to have a header, which means you have to duck under the header every time you get in the shower. I think Neo-Angles look good when they are tall. They are impressive.