Grid portfolio template will showcase your portfolio posts as a grid, same image size that you can set by yourself in the page editor. This page have their own options, such as posts per page, portfolio hover effect that will applied in each portfolio items in this page.
Image above is hover setting. Separated with Grid Options. Both of portfolio page have this options.
You can customize the item column per row, add a padding for each items. Choose the filter and load more styles. As you can see at the screenshot above, Grid Template Options have "Image Width" and "Image Height" form, that's mean you can arrange and insert your own size (adjust it with your smallest portfolio feature images).
Please note: You need to adjust the width and height as your minimum requirement size. If you use this template and your portfolio image size below the requirement (that you have set before), your image will not appear.