Too many questions about motor size. 
21xx (or super flat motor) is not going to magically turn you into a pro.
I also think that super-flat being better at throttle control is mostly a myth, based on a few high quality motors.

Motors in general can handle bigger prop and bigger quad, based on stator size (and thus weight).
Simple list grouped by stator size, and approx weight:

4g -100
1103, 1202.5, 1302

6g - 150
1204, 1303, 1303.5, 1105, 1403

10g - 220
1404, 1206, 1504

16g - 400
2004, 1805, 1606, 1408, 1507, 2104

20g - 500
2204, 1806, 2005, 2105

30g - 800
2306, 2207, 2505 

What is the point?
2104 is very small like 1507.  IMHO not worth it to upgrade from 2004 - almost same.

Why pick flatter?
A 1805 will have smoother bearing than 1507, and 1408 in particular might be poor choice.
But, at the extreme, 2004 or 2203 has low height, and much more likely to bend shaft or damage bearing.

Stator width to height ratio?
Lots of debate when pancake motor is it too flat.
2004 is already very flat, and 2104 is even more. 2303.5 is most extreme. I prefer 20xx to be 5mm.