This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
Yes, this is possible. However the procedure is extremely risky and should not be attempted if you cannot afford to lose the contents of your partition. Ensure that you either have a backup of the partition or are prepared to possibly have to reinstall everything.
First off run Defrag on your PC to collect all the files towards the start of the partition. This is best done from DOS, doing it from within Windows 95 most likely means that the shutdown procedure will scatter a few files about potentially causing problems for resizing.
Secondly use this BASIC program to change the size of the partition under RISC OS.
REM Resize parition file. chan=OPENUP("Drive_C") EXT#chan=<new size in bytes> CLOSE#chan
Change the filename to suit the name of your PC partition and adjust the size in bytes to the new size of your partition. Increasing the size of the partition is safe, but when shrinking the partition care must be taken to not shrink the partition below the size of the actual files contained within it.
Finally once the partition size has been adjusted boot up your PC and run the program PRESIZER.EXE. This is a DOS partition resizer and should allow you to now adjust the partition table and FAT to the new size of your partition.
N.B. It must be stressed again that this is a very risky operation and could easily result in total data loss of your partition. Also the procedure has only been attempted for increasing the size of the partition, not shrinking it - I'd be interested in hearing from people who have attempted to shrink their partition to know if it works.