For most DSLRs, size matters least because many are already around 2.7–3 inches. The more important differences are resolution and usability.

Higher resolution helps a lot when reviewing images, checking focus, and especially when using Live View with magnification for critical manual focus. A 921k-dot screen is noticeably better than a 230k-dot screen for that.

A vari-angle screen is most useful if you shoot from awkward positions—low to the ground, overhead, on a tripod, or for landscape and Live View work. It also helps reduce glare and makes composing more comfortable. If you mostly shoot through the optical viewfinder at eye level, it matters less.

Also important, and often overlooked, are brightness and viewing angle. A high-resolution screen is less useful if it’s hard to see in daylight or loses contrast when viewed off-axis.

So: resolution is usually the most important spec, vari-angle is a valuable convenience for certain shooting styles, and size is a minor factor. Paying extra is worth it if you use Live View often, manual focus on a tripod, or shoot from unusual angles. If not, a fixed lower-resolution screen may be fine.