Display Size

For in-person gaming, a bigger display isn't always better. Before deciding on a display size, consider the following:

Comfort

For any setup, whether you use the Material Plane hardware, touch, or just minis without tracking, there is a limit to how big a display can comfortably be. The reason for this, is that at a certain size, it'll become very difficult for players to reach across the display to move minis on the opposite side of the display. If it takes too much effort to move a mini, people might get annoyed about having to move their mini or might even make in-game decisions to reduce the amount of movement they have to make.

Before buying a display, I'd suggest doing the following:
Get some tape, preferably easy to remove tape (such as electrical or painters tape), and tape down a rectangle the size of the display you're interested in, preferably on the actual table you're planning on using. Then, see how difficult it'd be for your players to move their minis around everywhere within the rectangle.

Hardware/Size Limitations

Consider the size of the table you want to use. The display will have to fit on (or in) the table, preferably with some room to spare so players can still place down objects (minis, dice, phones, etc) on the table.

The Material Plane hardware also has size limitations, depending on the distance between the display and ceiling. Make sure the display isn't to big for the production or DIY hardware.

Map Availability

Some map creators design maps for specific display sizes. For example, a map that exactly fills a 42" display with a 25mm grid. If these maps are used on a bigger display, part of the display will be unusable. While if these maps are used on a smaller display, only part of the map will be visible.
Please note that the production Material Plane hardware uses 30mm bases, so a map designed for a 42" display will fit almost perfectly on a 50" display.

The most common display sizes you will find maps for are 40", 42" and 43", with some maps designed for 50" displays.