Hello Noemie

Unfortunately, at the moment there's no way to change the size of the form. You would be able to increase the font size using some custom CSS.

These are the classes used on front-end, and you can change text-size for each of them separately:

<style>
.amelia-app-booking input[type="text"], 
.am-modal.am-in-body input[type="text"], 
.amelia-app-booking input[type="email"], 
.am-modal.am-in-body input[type="email"], 
.amelia-app-booking input[type="tel"], 
.am-modal.am-in-body input[type="tel"], 
.amelia-app-booking input[type="url"], 
.am-modal.am-in-body input[type="url"], 
.amelia-app-booking textarea, 
.am-modal.am-in-body textarea, 
.amelia-app-booking select, 
.am-modal.am-in-body select, 
.amelia-app-booking .form-control, 
.am-modal.am-in-body .form-control, 
.amelia-app-booking .am-stripe-card, 
.am-modal.am-in-body .am-stripe-card,
.amelia-app-booking h1, 
.am-modal.am-in-body h1, 
.amelia-app-booking h2, 
.am-modal.am-in-body h2, 
.amelia-app-booking h3, 
.am-modal.am-in-body h3, 
.amelia-app-booking h4, 
.am-modal.am-in-body h4, 
.amelia-app-booking p, 
.am-modal.am-in-body p, 
.amelia-app-booking span, 
.am-modal.am-in-body span, 
.amelia-app-booking div, 
.am-modal.am-in-body div, 
.amelia-app-booking section, 
.am-modal.am-in-body section, 
.amelia-app-booking ul, 
.am-modal.am-in-body ul, 
.amelia-app-booking li, 
.am-modal.am-in-body li, 
.amelia-app-booking a, 
.am-modal.am-in-body a {
font-family:fantasy !important;
}
</style>

We insert font fantasy just for example, but you can use only some classes, and use text-size instead of font-family. 

Our developers are working on a new Customize section, so in the future you will be able to do this through the plugin.

Hi again Noemie

Please take a look at this article on WPBeginner.

This should explain in detail how custom CSS can be applied to individual pages, or on the whole site.