Start with stride length
Match the stride to your height: about 18 inches if you're under 5 ft 6 in, 20 inches up to about 6 ft 2 in, and 22 inches above that. A matched stride feels like walking. Two inches short, and it feels like a stepper you'll soon stop using. Machines under 16 inches are for shorter users and small rooms.
Front, rear and center drive
Rear-drive ellipticals, like Sole and Horizon, have the flattest, most walking-like stride and the longest frames. Front-drive machines, including most ProForm and NordicTrack models, follow a slightly rounder path and fit shorter rooms. Center-drive machines are the shortest and cost the most for the stride you get. Drive type matters less than the stride number and the frame weight.
Flywheel weight and how the stroke feels
A heavier flywheel, 20 lb and up, smooths the stroke so the pedals don't surge at the top. Light flywheels on budget machines feel fine at a stroll and jerky at a real pace. The Sole E25's 20 lb flywheel and 189 lb frame are the two reasons it should feel better than machines at the same price.
Check your ceiling and floor
An elliptical lifts you 10 to 14 inches off the floor at the top of the stroke. Add your height and a few inches of headroom, then check the ceiling; low basements catch tall users out. Length is the other surprise. A 20-inch stride needs about 70 inches of floor, and the pedal arms swing past the frame.