The 2048 x 1556 preset is generally speaking correct, as this is close to what most film transfer houses want to see (so they can work on it further).
Be aware that there are different formats on exactly what the active area of the film frame is, so that rule above bears some modifications depending on whether you use the Full Aperture (the whole area of the film strip, including the sound track area) or the Academy Aperture (where the sound track area is masked on the left side). The Academy Aperture is obviously what usually gets printed and it has an aspect ration of 1.37:1 or translated in terms of pixels: 2048 x 1492.
Now about the actual resolution you render at, it depends on how you want to frame your shots, as the people above already explained. The common standard for the so called “flat” films is 1.85:1 (pretty close actually to 16:9). However, the actual image area is still “4:3”, just with hard-matted black bars on the top and bottom.
Bearing in mind that for showing these flat films projectionists have to use a form of matting in the projector as well, you can imagine that it presents some alignment problems which could in the more extreme cases result in the showing of your precious new film misaligned either upwards or downwards by quite a lot, showing black on top or on the bottom if there is no content in that area.
That’s why a compromise is usually made: render at an aspect ratio of 1.66:1 to shave off some render time AND to still have a good error margin area in case of misframed projection. In short, that means rendering at something like 1828x 1101 (1828 comes from the fact that you want to take into account the aforementioned aperture/soundtrack problem, and 1101 from the aspect ratio) to get that desired 1.66:1 frame (however, always remember to design your composition for 1.85:1). If you feel courageous, you can also render directly at a 1.85:1 ratio by using 1828 x 988 (same explanation).
If you go for that widescreen look, you should aim for 2.35:1 (actually 2.39, and yes, there are so many SMPTE standards about that, you get a headache…). That one is achieved not by adding a matte on your image, but rather by rendering it “squeezed” in a (roughly speaking) 4:3 frame, which is usually for transfers taken to be 1828 x 1556 (with 2:1 pixel aspect ratio - your rendering software should have an option for that).
There is another 2.35:1 35 mm standard (but not really used for projection) called Super 35, which uses a wider area of the whole film strip and doesn’t require the anamorphic squeezing, so the resolution for that is 2048 x 872 (since it is used mostly for shooting film, it doesn’t have a soundtrack area to worry about). Don’t go there, as there is no practical use for what you want to do and there are also 2 very different standards on how to position the image (common top/center frame).
However, talk to the film recording house beforehand about all this (they usually have a brochure or a technical contact person which you can ask). Let me repeat that: talk to them, as you will either save some processing costs (if they charge setup fees) or you will avoid embarassing cropping or bad positioning of your image in the final print.
Hope this helped,