I don't want to get the cheapest. I want to define what a monitor / features you want and are worth to you. Do you run a fashion photography studio for a magazine? Or it's a hobby.
In my opinion, the best monitor to have is the one you can buy now and not in the indefinite future.
Yes, size matters. Connect your monitor for a "live preview" on your product photography studio? Is it closed? Do you use it for photo editing?
IMHO, the best size is a 27-28 "if you use a 16: 9 aspect ratio and when working on a desk.
But you could probably need a better quality monitor at the expense of size.
Some inexpensive monitors change color depending on the viewing angle. It is not good.
Any monitor from a decent brand will be "good enough" Benq, Samsung, LG, Dell, Asus, View Sonic, HP, Eizo … But there are specific features like the range of colors on specific series.
You don't need more FullHD on a 24 "monitor.
On a 27 to 28 inch monitor, you don't "need" more, but it can be a nice feature.
But here is the really important feature to look for when you think of a "monitor for photography". Color space.
To date, (2020-01) the Benq website has the most useful classification of monitors with a targeted audience.
Specialized monitors for photography (and design) are those who have a wider range of colors. There are several relevant spaces. The bigger the better.
For photography, the main ones are sRGB and AdobeRGB. The ProPhoto is currently too large to be covered with current technology to my knowledge.
For the video. sRGB, Rec709, NTSC, DCI-P3.
Some photography monitors claim to cover 99% of AdobeRGB. Use this information as a reference.
This is a subject that needs to be deepened:
Do searches like this, combining and comparing color spaces to find more information:
Some unimportant features
Refresh rate. If you are not a player, it is not important.
Curved or not.
Bright or not.
Led or not. This may have some effect on the black point, but it is not a function to look for, but the overall result.
Integration with GPU. It is also a feature for gamers.
Cost vs quality.
If you want to have a professional workflow, but limited on a budget, choose in this order.
Wider color space possible
In addition to a monitor, you can consider a calibration device like ColorMunki or ColorSpyder.