Once you open your image in Pinta, click on the Image menu and then click on Resize Image... It will bring up a dialog box that will allow you to resize your image via a percentage value or an absolute value where you enter the height and width. I have illustrated steps on my website below. Just scroll down to the Resize Image... section.

Pinta: Image Menu https://learnubuntumate.weebly.com/image-menu.html

I hope this helps.

Good luck!