Which app did you use to customize the form page, so I can give you the right strategies you will use
I used Klaviyo
Okay
Follow this step
Go to Klaviyo Dashboard → At the left side of the dashboard you will see Sign up form, Click the Signup Forms.
You will see the list of pop up form you have created
Select the form you want to edit.
Click on the Form Container (the outer box).
Adjust the Width (e.g., set it to 300px–400px instead of full screen).
Be previewing it as you are making the changes
Reduce the Padding & Margins under the Design tab.
Click on Text Elements and reduce the Font Size if needed.
Save and Publish.
Let me know if this help
Did you design the pop up form by yourself
Greetings dear merchant!!
I would recommend you using klaviyo for your preference! 
I actually don’t see this popup in Klaviyo. This is just the button that pulls up the cart.
So I actually guess it’s from UpCart. The sticky cart. I don’t see a way to make it smaller though
Depends on the plugin you used to create the form
I was using upcart
Then Check your website maybe you have activate Auto pop up form in the customization page of the website
I was using the UpCart. It’s labeled as “sticky cart”. I just don’t see a way to change the size.