steam_get_quota_free();
Returns: Real
With this function you can find out how much free space is left
for the user of the Steam Cloud quota. The value returned is in
bytes.
if steam_is_cloud_enabled_for_app()
{
quota = steam_get_quota_free();
}
The above code checks to see if the steam cloud is enabled for the game and if so it gets the size of the free storage space and stores it in a variable.