how to change login or register page font size?

You can edit the HikaShop CSS in the config under the tab display in the CSS zone. There, you will be able to add some CSS code to change the font size.

I told you where in my previous post: You can edit the HikaShop CSS in the config under the tab display in the CSS zone.

I was saying that the interface to edit your CSS is in the "display" tab of the configuration.

You can add something like that in the CSS file:

.hikashop_checkout_page h1{ font-size: 1em; }