At this time, there's not a way of changing the size of the image in QuickBooks Online. I recommend trying to adjust the size of the image outside of the product then upload it to QuickBooks.
I can see how this could be beneficial to you and your business. If you'd like, I can submit product feedback to our developer team, so this could be considered in a future update.
Please don't hesitate to let me know if there's anything else that I can do for you. I'll be here every step of the way. You can always reach out to the Community or me anytime you have questions or concerns. Take care and have a good one. You certainly deserve it!