There are a few things you can do, varying in degree of difficulty:

* Ensure a minimum area for all prints / all layers - even if it requires adding a 'sacrificial' second cylinder or tower to the print plate before slicing. You'll get a feel for when geometry is too small and needs a tower. Spacing the tower some distance away from the main print can help to increase time a bit without using any more plastic...

* Slic3r has settings to slow down printing under a certain layer time - this only helps so much, as the nozzle is still touching the print and like a soldering iron it keeps it hot. This has a minimum speed setting, but reducing this too much will not help - same reason! If you are keen to experiment a bit, you may be able to add a routine to the slic3r "layer change" gcode to move the extruder off the print, idle for a few seconds, then return (switch to relative coordinates so you don't lose your spot!) - I can't promise this will not affect print quality (it could add lumps or strings, potentially). You could set this as another printer profile in slic3r, so you can reuse it when you have small prints. Here's a draft of some layer change code:

G91 ;set relative coordinates
G1 Y-50 F3000 ;move back 50mm (assumes you have room!)
G4 P10000 ;wait 10 seconds
G1 Y50 F3000 ;move back to original location
G90 ;back to absolute coordinates

* Add a nozzle fan to the printer - this blows on the print while printing overhangs and small features (or small objects), helping them cool faster. This is a bit of an investment in effort, but will allow you to print smaller objects than you otherwise could, and is pretty much necessary if you want to print in PLA.