I would stick to something in the 20g-30g range for a first tank. My reasons are below
1. Price - not a huge initial investment to see if you like the hobby and want to continue. Also equipment needed in this range isn't budget breaking, and plenty of "generic" options that work well
2. WATER CHANGES - A simple 5g water change can solve many problems in this range of tank. When you get larger tanks, you need the supporting equipment to go with it
3. Time - It takes substantially less time to care for a 20g tank than a 120g tank.
4. Fish options - Lots of unique fish options work in this range. No, tangs wont work, but lots of other things will
This video does a great job of highlighting what a great nano tank can be, vs a much larger tank.