For which Geeklog theme are you talking about?
Most themes use a standard sized font for the main text (like the themes included with Geeklog). If the user has trouble seeing then the best thing to do for the user is to modify his browser zoom setting. This way all websites will appear larger (and not just the Geeklog one).
Windows 10 also has a feature (which I use) that you can change the Scale and Layout of the text for all apps (including the internet browser). I have mine set at 125% on my laptop since the screen is smaller than my desktop. You can search for this feature in the Settings.
One of the Geeklog Core Developers.
