First I would ask the situation of why you need to rotate the video, and how you intend to deliver.
Here is one solution:
Choose a Sequence Preset that meets your basic needs (1080, proper frame rate) then click on Settings and choose CUSTOM at the top. Now change the 1920x1080 to 1080x1920. Put your clip into timeline and rotate 90 degrees. Export with "Match Settings". Just tried this, it works to create a "vertical video" clip at 1080x1920.
Another would be to Scale the video down to fit the 1920x1080 frame after rotating, with the understanding that you will have a small video surrounded by black on the sides. But if this is something you want to post on YouTube for instance, I don't think they will be set up to accept/display 1080x1920 vertical format described in first solution, so you may have to live with the black border solution.
Safe Harbor Computers
Thanks for your feasible-sounding suggestion (I haven't yet tried it out). I need to rotate the video because I'm shooting a long, thin hand-held control device as part of a work project. This controller has an LCD screen with some very small text and graphics, and I'm shooting very close to it with the camera tipped over at a right angle to maximize the size of the image in the camera's frame. The goal is to maximize the resolution and on-screen size of the small, not terribly distinct details on the controller's screen. (Actually, the visibility of the screen's details is perfectly fine when you're looking at them with your eyes. It's just that they don't come across terribly well when you video record them.) I had no problem using Microsoft Movie Maker to rotate the entire video frame, but the video formats into which Movie Maker will export the video are much too low-res. Hence, I'm using Premiere Pro, which rotates the video with no problem but leaves me with a truncated image.
I expect the solution you recommend will solve my problem, but hadn't previously considered the possibility that YouTube might not accept video with a 1080 x 1920 aspect ration. My client hasn't decided on a distribution medium, but since YouTube is one of those under consideration, it wouldn't be a good idea to pursue a production strategy that would rule it out. Unfortunately, when I shot the controller with the camera in its normal orientation, the detail that needs to be shown comes out too small.
I assume that your recommendation was that I first open the Sequence Settings dialog box, which I did, but I didn't see any Custom tab or anything labeled Custom anywhere in the box. I read some additional Help files about changing sequence settings, altering the frame's aspect ratio, etc., but none of the articles I examined led me to a clear solution. Perhaps I misinterpreted your suggestion, in which case I would ask you to explain it in a bit more detail. I'm a total neophyte as far as using Premiere Pro, so please bear with me if I seem a bit dense.
When in the New Sequence Panel, the "Sequence Presets" tab is the first one active. Click on "Settings" tab to the right of it. At the top, under "Editing Mode", select "Custom" from the drop-down menu. Then you can change parameters such as frame size and frame rate and such as needed.
I believe I understand what you want to accomplish with your workflow. Might be helpful to a see a frame grab of the video to be sure, unless it is proprietary material.
I think the big question is can you deliver as 1080x1920, that will be the deciding factor. If not, then perhaps use a smaller 16:9 frame size such as 720p (1280x720) and rotate and size the 1080 material into the smaller window. You may then be able to maintain more detail/readability when rotating the video orientation.
Thanks to the additional detail in your last message, I finally figured it out, and the procedure worked great. I also investigated the question of what aspect ratios YouTube will accept and found that nonstandard ones like 1080 x 1920 are O.K. YouTube simply adds black boxes on either side to fill out their standard 16:9 frame.