When choosing a portable monitor, it all hinges on your use case and to what device you are connecting. The very concept of a portable monitor makes most sense with a USB-C equipped laptop.
That’s the most reliable way to connect a portable monitor and ensures both compatibility and plenty of bandwidth for any resolution a portable monitor is likely to offer. That said, many portable screens also support old-school USB-A connectivity. Just be aware that will require software and drivers, which could present a problem depending on the device you are using to drive the display.
For most mainstream Windows PCs, that should be fine. For everything else, it could be more problematic. Running over USB-A can also limit resolution and colour fidelity due to lower bandwidth. That said, some portable monitors support micro HDMI and even VGA.
Speaking of bandwidth and resolution, most portable monitors are 15.6-inch panels with 1080p resolution of 1,920 by 1,080 pixels. But more compact models with 11-inch screens are available.
Battery or no battery is another key question. You can get portable screens both with and without. Screens without a battery are cheaper and lighter. However, if you’re using a portable screen away from the mains, they will drain your laptop’s battery pretty fast.
The final really important factor is brightness. If you are planning on using your portable display outside, you want as much of that as humanly possible. Most are quite limited in that regard, only topping out at a little over 200 nits. Aim for the brightest you can get while taking into account that brighter screens will use up even more battery when powered by your laptop.