ASUS ROG Strix 17.3" 1080P Portable Gaming Monitor (XG17AHPE) - FHD, IPS, 240Hz, Adaptive-Sync, Built-in Battery, Smart Case, USB Type-C, Micro HDMI, For Laptop, PC, Phone, Console, 3-Year Warranty

I have it. It has great pixel density. Also has a built-in battery that helps out a lot when doing low wattage on Rog and 60hz out of the screen.

Can it truly be called "portable" above 17-18 inches... I got the asus 550$ one, is overpriced but does a good job

I use the Asus ROG 17 inch portable gaming monitor: https://rog.asus.com/us/monitors/below-23-inches/rog-strix-xg17ahpe-model/ It has a battery on-board that lasts 2-3 hours. When you connect it by USB C, it will give you the option to pull power from the computer or to stick with the battery. This should allow you to max the battery time on your Flow.

I've got the ASUS ROG Strix XG17AHPE - LED portable monitor. It's 17.3 inch which is perfect for me to work and game on. Unfortunately my mini-pc doesn't have a Thunderbolt with display access lol, so I always need two cables :(

This is the one I use and it's 100000% worth the money

In my experience with the ASUS XG17 it looks absolutely fine at 1080p and up to 240Hz. I don’t think there are many people with the eyesight to need 1440p or even 4K at that size.