I own both a macbook pro retina 15inch(late 2013,intel i7 4850hq with gt 750m 2gb gdr5) as well as a decent gaming rig(i7 2600k, gtx 770 4gb each 2 way sli). And i would like to say that retina macbooks are worth. But again u can't game on them, as they are not meant to be toys, for example when i fired up mass effect 2 on my mac, the cpu temp reached an astonishing 91c(celcius) mark, which is not good for a laptop i guess. But for work it is worth every penny and i highly recommend macs for studying and doing everything else excluding gaming.
About 15 inch and 13inch, i can't tell you which would be suited to your need, but 15 inch packs more power as well as can display more pixels on screen making it a bit more beautiful. Macs are extremely lightweight so i guess 15 inch won't give u much headache. I would suggest go for 15 inch if u r not an international celebrity who has to travel a lot. But again choice is yours.