Uninstall.

Yes it will.

Only if it doesnt support steam cloud, if not you should manually make a backup of it

I always wondered how much time your save is kept on the steam cloud.

Only Steam cloud game won't have save deleted. But in other games, saves save separately to the main game folder so possibly not too, but with Steam cloud it will save even of you change your PC. You have to say what game is, if is Ubisoft or EA game forgot about Steam Cloud in them XD D;

Sometimes yes, sometimes no. If save data is stored in the Steam folder only, then yes. (Unless it is also on the cloud.) Some games that store save data in your documents folders or AppData folders won't lose their info. (You can also backup almost any sort of save data by finding the appropriate folders, and keeping track of where to put them when you re-install.)

I always back up games I'll return to, so uninstalling is no problem.

Here you go: Click

Just back your saves up before deleting, whether it uses steam cloud or not :)
Then stick them in a RAR file, and voila! Doesn't take much HDD space :)

Fear 3 saves all upgrades and saved games in the Steam folder.
So if you uninstall it you lose everything and have to start at level 1.

Depends if the game is supported by steam cloud. If it isn't well your save are probably gone forever.

save game in "my document" folder won't be deleted

If on steam and it has Steam Sync: No
Some games save in documents and it will stay there unless deleted (reinstall of OS / user)
Its stored inside the game folder and it goes away with the installation.

I may be wrong, but it depends.