To get the same dimension, take the height of the large monitor and match the width of the smaller monitors to that. Make sure that can rotate a quarter. The cheap models usually do not support that.
To make the effective resolution identical look for side screen with a resolution of 1080xsomething.
Also try to get screen with IPS panels for the side monitors. That way you get a good viewing angle. Avoid panels based on TN panels for all configurations where do not like look straight at the screen.
(In the picture you provided you would look reasonably straight at the screen, however IPS get more important with the size of the screen and with larger angles. It also has the advantage of producing better colours).
Some more detailed information:
A modern monitor usually comes with a panel based on one of these three techniques:
Twisted nematic (TN) displays have the following qualities:
- Fast screen drawing (which used to make then better for gaming, though the other two techniques have caught up and are also fine for gaming. A 15 year old IPS panel on the other hand might not suffice for gaming. But unless you are going to buy ancient monitors you can forget about this)
- Poor colour quality. (Do not do graphic design on these. For gaming it is fine).
- A not all that great horizontal viewing angle and colour differences depending on the angle. This is visible in the picture I took. If you are looking directly at a small or medium (say up to 20") screen this is not a problem. The display shows in the picture is a 26" at a small angle to my main screen and here it already become visible.
- A poor vertical viewing angle. Usually not a problem unless you rotate the screen. If you rotate 90 degree then you effectively swap vertical and horizontal angles.
- Cheap to produce in on direction. (e.g. make a long/wide screen). This sells great.
PVA/MVA (Patterned vertical alignment / Multi-domain vertical alignment
- Better colours and better viewing angles than TN
- Good colour.
- Good viewing angles.
- Usually more expensive.
Note that a good TN panel can be better than a poorly build IPS panel. Takes these as general guidelines. Read reviews before buying anything.