What size can I add images?

I am not sure if I understood your question correctly. You can add an image of any size to your form, but there could be an issue with the load time of a form. I tested your form the load time is more than 7 seconds on my end. The issue is related to your logo image. I would suggest you to replace it with a smaller size image to reduce the form's load time. 

If your question is related to the size of the images, your users will be uploading to your form, then please check this guide. 

Changing the File Upload Size Limit