I don't know what a HTML preview would look like on a high-resolution screen like yours, and I'm not aware of a permanent zoom setting that you could use, but The Bat's makers seem convinced that it works well as-is. They adapted the program to hi-res screens a few years ago, at version 7.4, of which they said:

"We have made significant changes to make The Bat! v7.4 look on ultra-high-definition 4K monitors as good as it does on ordinary ones. The users of regular monitors who have configured Windows to scale the user interface, for example, by 150%, will also benefit from these improvements in The Bat!"

Which screen division mode are you using in The Bat? (Workspace | Window Split Mode)   Personally, I use and recommend "full height account tree" which gives me a small, full height Folders panel on the left, and on the right I have the message list on top, occupying about 2/5ths of the window's height, and the current message below it, which takes 3/5ths of the height and so makes the biggest section of my screen. This way I never have to 'open' messages to read them; I can simply scroll through the message list and read them.

I view all messages in txt format by default, but I have a small html-tab at the bottom of each html message to switch to html view. There are two ways to view html: securely using The Bat's internal interpreter, or through Internet Explorer's interpreter (Options | Preferences | Viewer/Editor | HTML Viewer).

This doesn't really answer your question (I don't think there really is an answer) but maybe it will give you some ideas of things that you could try.