Here's a procedure I use with PS 5.5. It's a few years old from the old Andy Art site which is now part of screaming something or other. There are probably better ways to do it now but this works okay as an old *Photoshop only* solution.
1. In the Channels palette create a new channel.
2. Use the marquee tool to draw a rectangular selection to whichever size you need. Fill the rectangle with white.
3. Select all. Select Filter>Blur>Gaussian Blur. Start with a setting of 5 and play around with it until you get the 'roundness' you want.
4. Adjust the levels - CTRL-L - to 120, 1.0, 140. That's your rounded selection.
<<There's another half-step somewhere along here but my machine is going buggy now (poor thing worked hard today) and I can't start Photoshop right now but if I can figure it out anybody can>>
5. Go back to the main palette and load the selection you just created with CTRL-ALT-4. From here you can fill it, stroke it, paste something into it, whatever.
Seems clunky but actually quite simple once you go through it once or twice.