Hi,
1. Just turn on the "required" option of the custom field and it will be automatic.
2. You can do that with CSS on your website by simply forcing the height of the image. The exact CSS will depend on how you configure the field and where you want to force the size of the image.
For example, if you create a custom field called "image" and of the table "order" and that you want to force the image size on the checkout, you can use that CSS:
#hikashop_checkout_additional_info img{
height: 100px !important;
}