Well, the free version of Titanium Backup will show you how much room you have on /system.
And I'm a HUGE fan of DiskUsage (look on Play Store) to find space issues... it gives a graphical representation of files so you see where the space is getting chewed up. The only hangup is that to fix/look at system stuff in place like that, you need root access.
If you don't have root access, or can't.... AND your system partition is crammed, then you have to flash your factory image... You have an Ace 3, so there SHOULD be a factory image out there that you can flash, along with instructions on HOW to flash it.... I had a SIII back in the day and it used a tool called Odin... and it was pretty easy to do.... not sure if that will apply to your Ace though.