If required plugin "Intuitive Custom Post Order" is installed and activated, you are able to change posts order manually. It's a definitely big help in dealing with portfolio grid, because changing post date to reorder posts in the grid is quite frustrating. But there is a new unique feature that's even better.
If you already added a few portfolio posts and created porfolio page, navigate to Appearance ▸ Customize.
Open your your portfolio page in the Customize preview window. As you can see, every portfolio post has a buttons block.
Let's review a full set of available options. It depends on portfolio mode selected. Here is a "Mosaic" mode activated on the screenshot above. First two buttons (arrows) extend the functionality of "Intuitive Custom Post Order" plugin (it should be activated to enable these buttons, by the way). The next holder allows to modify width. And, finally "1x", "2x" and "3x" buttons are there to change item height. Summing up, you can just use a perfect options combination for making quick changes in the portfolio layout.
"Masonry" mode lets you change items order only, because all items has auto height and fill 4/12 of the page width (3 columns). Within "Flex" mode you can also modify item width, but height also remains original.