Is it "on the browser" or "in the browser"?

As a native Spanish speaker, I often get confused by the correct usage of on and in. In the following examples, what would be the correct way of referring to the font size for characters used by a web page and why?

Use the correct font size on the browser.

Use the correct font size in the browser.

Thanks.

Both will be understood easily enough, but I would say you actually used the best option (for your specific example) in your question:

Use the correct font size for the browser.

More generally, it's slightly more correct/common to use in when referring to the context of a computer program. This is because the program has an interface (for a browser, its tabs and controls) and the thing you're describing is contained by that interface.

By contrast, when referring to a particular website, on is more common, e.g. "on Facebook" or "on English.SE". Also, people who are using a computer are usually described as "on the computer" as opposed to 'in' or 'at' the computer.

With a web-browser, it's "in the browser". The viewing area is contained within the frame of the browser. If you have a google toolbar, that is "on the browser"; placed on top of the viewing frame.

I undertand that the word 'en' is used in Spanish for all prepositional refrences such as on, in and at however while speaking English, its appropriate to use 'in' in the example you just used