Remove horizontal scrollbar

The text is likely smaller because your new laptop has a higher resolution screen than your old one.

You could try changing the windows screen display settings to 125% (assuming it's 100% currently) and see if that helps. But it will affect every program on your computer. Do note that having Windows at 125% may cause other display issues depending on the applications you use.

And if you generate PDF files, you may find the PDFs don't generate with the right font - you'd need to switch back to 100% to before generating the pdf in this case.

I thought perhaps part of the problem was sight issues but obviously not if it was OK on the previous machine and others around you.

There are some Windows settings but they would affect everything on your machine and I'm guessing it is only RoboHelp that is the issue or you wouldn't have come to this forum.

I found some posts about this, but because I did not have this problem on my previous PC (and neither do my colleagues), I was hoping it was just a setting on the new laptop. 

For now I've changed the CSS, this is a workable solution for me. Thank you.

Sorry but the answer is you are stuck with it. A search will find other posts asking the same question but no solution was ever found to the problem when increasing the magnification.

You have also asked about making the text bigger. There might be a solution there. Create a copy of your CSS. When editing you use one CSS where you have set the fonts to be larger. When generating an output you can select a different CSS to override that, there you would use your current CSS.

It's not elegant but it might work.