The size of text is determined in part by the source, the web page you are reading, but also by the settings of your browser. Far too many people never bother to look into and tinker with the great variety of settings that their browsers allow to be set or changed, and default font size is an important one.
I only recently had my optometrist change the prescription for my video glasses (which, for most spectacles wearers, are somewhere between "distance" and "reading" prescription levels) to something notably stronger, but I still find an awful lot of web pages whose makers seem to think we are using Sherlock Holmes style magnifying glasses to view their work.
For Firefox, one clicks on--
Tools --> Options... --> Content --then sets the "Size" roller datum. I use 20. One can also click on "Advanced" and decide whether to allow pages to set their own choices (one's own choice, in my case 20, is used only if the page does not expressly set a font size, unless I choose to not allow pages to set their size; I prefer to let them, then fix it as needed).
Again for Firefox, highly useful is any "Add-On" (such as PrefBar) that allows simple click-on changes to font size.
Also, postings often allow choices of font size, as this board does:
All I can say is that on my screen, at the size I read this board, a serif font like Times New Roman looks worlds nicer and easier to read than the default sans serif.