Jumping spiders live vertically. In the wild they hunt on tree trunks, fences, and walls, and they build their sleeping webs up high. So the first rule of housing one is that height matters more than floor space. A tall enclosure lets the spider climb, anchor a web near the top, and hunt downward the way it naturally wants to.
For an adult Phidippus, something in the range of 20 to 30 centimeters tall works well. Bigger isn’t automatically better, because in a very large enclosure a small spider can have real trouble finding its food. Slings do better in small vials or mini enclosures until they’ve put on a few molts.
Ventilation matters more than most new keepers expect. Stagnant, humid air is the most common thing that quietly kills jumping spiders. Look for cross-ventilation, meaning openings on at least two sides so air actually moves through. All of the enclosures I build follow this rule, because I got tired of seeing spiders kept in sealed jars.
The members version of this guide includes the exact dimensions, vent layout, and lighting I use on my own racks. Morbid Club members can read it now.