I've just returned my 13" tbMBP for a 15" tbMBP... for me the 15" is still portable, but the added screen size just makes it that much less of a strain on my eyes.
I originally was sure the 13" was perfect, so it's best to try both and make you decision.
I did the exact same thing. Bought a 13" nTB base model. Loved it actually. Performance was more than sufficient for what I do.
But, then I realized that I'm almost always going to just take my 12.9" iPad Pro to work with me, which for what I need a personal device at work, it can do virtually all.
I usually don't need the MBP at work since I have two desktop machines at work (a triple head Xeon workstation that would make any nerd like me jealous) and a 3.0GHz i7 Mac mini I bought 18 months ago for myself also connected to one of the Xeon's displays to handle any Apple-centric stuff.
The 12.9" iPP and the 13" MBP have virtually the same screen size & resolution. And I can actually do a bit more work-related stuff on the iPad (complete/sign documents with the Apple Pencil is a big one, and iOS devices can connect directly to my enterprise's Outlook email server where as personal laptops have to go through Citrix).
So, since the laptop wasn't coming with me on any kind of regular basis, did I need the svelte 13" MBP or could I go for the big, gorgeous screen on the 15"? Besides, for eye comfort I was running the 13" usually at 1280x800 hiDPI (with occasional 1440x900) whereas I can run the 15" at 1440x900 hiDPI for more space at the same comfortable text size and flip to 1680x1050 when needed and still not strain too much.
Even though I have a decent desktop at home (a 2013 i7 27" iMac at 3.5GHz), I prefer to work on the laptop from the kitchen table so I can have conversation with my wife instead of being hidden away in the home office downstairs.
So ultimately I exchanged the 13" nTB for a 15". And the new 15", being just 4 pounds, is only 8 ounces heaver than the 2013 13" rMBP I had before.
I still think the 13" nTB is a fantastic machine, and I'd keep both if my wife would let me... but alas...