Can I use my late 2013 MBP with Dell2515H?
You can use 4K displays and Ultra HD TVs with these Mac computers:


MacBook Pro (Retina, Late 2013 and later) 
Mac Pro (Late 2013) 
iMac (27-inch, Late 2013 and later) 
Mac mini (Late 2014) 
MacBook Air (Early 2015) 
MacBook (Retina, 12-inch, Early 2015)


So yes, your MacBook will support a 4K external monitor. You can decide how big you would like your interface/text etc. to appear in system preferences the same way you can on your internal monitor.

Source: Apple Support Documents