Under-bed boxes that slide out without a struggle
Clearance beneath the frame and space beside the bed are equally important.

Find the lowest obstruction
Measure from the floor to the lowest part beneath the bed, including crossbars, brackets and centre supports. The clearance at the visible edge may be greater than the space farther inside. A box that slides in halfway is not a useful fit.
Include wheels, lid clips and handles in the box dimensions. A soft case can compress slightly, but it should not be packed on the assumption that it can be forced beneath a frame. Leave room to move it without scraping the lid or damaging the bed.
Measure the route out
The box needs somewhere to go when pulled free. A bedside table, wall or wardrobe may limit the available direction. A long box can fit under the bed yet be impossible to remove without moving furniture. Mark its outline on the floor beside the bed before ordering.
If it will hold heavy items, think about the full weight and the grip. Small wheels can help on one floor and catch on another. Check whether the design suits your surface and whether the handle is intended for the load. Do not lift a filled box by a decorative lid tab.
Store what can wait
Under-bed storage often works best for items used occasionally. Putting daily clothing there may mean kneeling and pulling out a box every morning. If that movement is uncomfortable, use the space for spare bedding or another suitable seasonal group instead.
Keep contents clean and dry before closing the container, and choose materials suited to them. A sealed plastic box does not fix a damp room. Check the area periodically for moisture and dust, especially if the bed is rarely moved.
Label the visible end rather than the lid alone. Two matching boxes are easier to distinguish when the information can be read without pulling both out. A simple list of broad contents is enough; avoid turning the label into another task that must be rewritten for every small change.
Before buying several boxes, try one in the most awkward position. Pull it out with the room arranged normally and the bed made. If the task feels cumbersome when the box is empty, a full set is unlikely to make the arrangement more convenient.