Where should I put (or should I use) a giant button?

I'm a mobile developer and I'm working on a pet project that has a giant button. Because this button needs to be pressed while the user's hands are likely occupied, big seems good to me. However no matter what I do it just looks like a giant ugly button.

The button kind of centers around the use of the app, and will be pressed frequently.

So I'm thinking about how to put this button somewhere. Currently its a giant button on the bottom of the page. The user also needs to be able to see the majority of the screen, so its a transparent button.

A couple of ways I'm thinking:

-Large plus sign in bottom right. Tap near it and it performs the action.

-Just tap the screen in general, screen flickers or pulses to acknowledge the tap. The user can hit another button to unlock the screen for the less common interaction. Though this seems interesting, it is extra work for the other functionality (they need to be able to zoom/scale/position an image as necessary). I'm not sure the trade off here is worth it.

-a faint, but visible, outline of where the button is. On load, it will be visible, then fade, to show the user where the button is. When touching the button, it should flash into view momentarily to provide feedback that it was tapped.

Anyone have any advice on how to deal with a large, central, user interaction?

How large should a button on a mobile device be if its expected that it may often be hit by a pinky finger while the hands are busy? Bonus points for consideration under the material design umbrella.