If you're treating legs, arms, underarms, and bikini line rather than just a small patch, the single spec that affects your day-to-day experience more than any other is flash window size - the surface area of the device's business end that contacts your skin with each pulse.
Why Window Size Matters More Than It Seems
A small flash window (roughly 1-2 cm²) might be perfectly fine for facial hair or underarms, where you're covering a small, contained area. But scale that same window size up to two full legs, and the number of individual flashes required multiplies dramatically - often into the many hundreds - which directly increases session time, the number of times you have to reposition the device, and the likelihood of accidentally missing small strips of skin between passes.
Larger flash windows (commonly in the 3-4+ cm² range, with some devices advertising "XL" windows around 4-9 cm²) cover more skin per pulse, cutting session time substantially. A device with roughly triple the window area can treat both full legs in a fraction of the time a small-window device needs, all else being equal.
The Tradeoff: Precision vs Speed
Larger windows are faster for big, flat areas like thighs, calves, and forearms, but they're clumsier for contoured or small areas - ankles, knees, underarms, and especially the bikini line, where a large flat window can't make full, flush contact with curved or small surfaces. This means the flash may fire without proper skin contact in spots, wasting a pulse and potentially leaving gaps in coverage.
The better full-body devices address this by offering two attachments or a window that can be reduced in size, or by shipping with a dedicated smaller cap for precision areas. If a device advertises a large window as its main selling point, check whether it includes any accommodation for smaller or curved areas - otherwise you may end up needing a second, smaller device for the bikini line and face.
Other Factors That Affect Full-Body Session Time
Pulse mode vs single-flash mode. Many full-body-oriented devices include a "glide" or "pulse" mode that fires automatically at a fixed interval (often every second or faster) as you slide the device continuously across skin, rather than requiring you to lift, reposition, and manually trigger each flash. For large areas, this cuts session time dramatically compared to single-flash mode and is arguably as important as window size itself.
Recharge/recycle time between flashes. Some devices need a brief pause (a fraction of a second to a couple of seconds) to recharge their flash capacitor between pulses. A large window paired with a slow recycle time doesn't save as much time as it appears to on paper.
Ergonomics for extended sessions. A full-body session run at even a large window size can take 15-25 minutes. Handle weight, grip shape, and whether the device gets uncomfortably warm during extended use all affect whether you can comfortably finish a session in one sitting.
Realistic Session Time Expectations
With a large flash window and glide/pulse mode, both full legs can typically be treated in roughly 8-12 minutes; a small-window, single-flash device can take three to four times as long for the same area. Arms, being smaller in surface area, take proportionally less time regardless of window size, so the difference is less dramatic there.
What to Prioritize When Shopping
If your main use case is full legs, arms, and torso, prioritize window size and the presence of a glide/pulse mode over other features - these two specs will affect your actual weekly time investment more than flash count or peak energy rating. If you'll also be treating the face or bikini line with the same device, check for a reduced-window attachment or cap before buying, rather than assuming the large window will work everywhere.
Bottom Line
For full-body use, flash window size and glide-mode capability are the specs that most directly determine whether treatment sessions fit realistically into a weekly routine. A device with a large window but no way to scale down for curved or small areas will save you time on legs while frustrating you on the bikini line - look for both capabilities in one device rather than optimizing for size alone.