How and where can I increase the font size on the order form?

Hi,

This usually depends on the CSS of your Joomla template.
I would need a link to the shop to be able to check the situation in order to tell you exactly what to do and be sure it works.

As a general solution, you could probably do it with that kind of CSS:

#hikashop_checkout_page select, #hikashop_checkout_page input {
 font-size: 20px !important;
}

Hi,

It's going to be hard to say anymore without a link to the shop in order to check the situation as your template's CSS might be interfering.
Could you provide it ?

Hi,

Then I'm afraid we'll have to wait you move it online to be able to check the situation.