Travel monitor for photo editing

I can think of three options. 

• Very small TVs. They can be affordable and may be available in HD (1920 x 1080) resolution. However, TVs are made for casual consumer TV watching so image quality and color accuracy/gamut may be questionable. Standalone computer monitors are not usually available in travel-friendly sizes. 
   
• Tablet computer set up with specialized software to make the tablet a second display over a network, like JJMack already said. If you happen to be using recent Mac and an iPad, they work that way out of the box using the Sidecar feature. Another variation is using Luna Display hardware to make an iPad a second wireless display, which was recently upgraded to work with Windows PCs (Macs were already supported). 
   
• You could also get a field monitor designed for pro video work. Those support HDMI, which many Windows laptops have. They often cost more than a small TV, but several reasons could make them worth it: They are made to be portable and compact, and because they are meant to be field monitors they have a much better chance of being reasonably calibrated to Rec.709 (sRGB).

If your Laptop is a Windows Laptop you can use any good quality  Android or IOS tablet and use spacedesk device driver to attach the table as a second display  via a network connection Wired would best. wireless will also work.  The machine may need to be on the same local network. I do not think you can color calibrate your tablet for Spacedesk driver or for IOS or Android.   I think the color would be the ones IOS or Android support.   There are some small cheap portable displays. However I feel having an IOS or Android tablet is more flexible and no wires are required.