.SIT is the "default" file extension for compressed Mac Classic files.
To expand .SIT, Stuffit Expander is needed.
One possibility is this page, 3. DL:
http://macintoshgarden.org/apps/stuffit-expander-55Unzip the file in Windows and add the resulting Toast image to your SS Volumes.
The Toast will be mounted at boot time and you may install Stuffit Expander onto your boot volume.
Please note that Stuffit 5.5 will expand a lot of .SIT files, albeit not all of them.
Recent versions of Stuffit can create .SIT files incompatible with 5.5.
In such case, some kind soul will surely re-package a desired file for you.

In any case, Stuffit files MUST be expanded on a Mac volume large enough to hold source and target.
Expanding does not work on the Unix or "This Computer" volume, nor with files moved from there to the Mac desktop.
Some large disk images are collected here:
http://macintoshgarden.org/apps/disk-images-emulators