Overall an enjoyable game! I liked that you added upgrades to the player after the rounds. Here's some feedback since you requested it:
- Move the HP indicator to the center, it took me quite a while to find it ^^'
- I believe the powerups can only roll the size one, I didn't encounter any other upgrades in my entire playthrough?And another thing about the size up: It moves your gun further up, so the bullets spawn further up as well. With 1 size upgrade you can only headshot the zombies anymore as their arm hitboxes are too low for the bullets to connect with them. If you manage to get 2 of the size ups it's impossible to shoot the zombies anymore :P
Overall this is a fun a little game though and I had a good time playing it!