Can I make my HUD take up less space?
Solution 1:
The HUD is relatively big compared to other MOBA games such as LoL. There is no "legal" way to resize the HUD apart from taking a HUD-skin which might be a teensy bit smaller.
It took me a while, but you get used to it.
Solution 2:
The HUD in Dota2 is set to a constant size to give everyone a level playing field.
In some games the higher the resolution, the more you can see. Which in turn gives an advantage to the players with the better tech.
So for competitive/balance reasons, the HUD cannot be re-sized.