What's the 2001 Dodge Ram 1500 brake line size?

Regardless of configuration, the 2001 Dodge Ram 1500 will always have 3/16" brake lines. It's technically possible to upgrade sections (or the entire system) with larger diameter brake lines, but it's beyond me why You'd want to go through the trouble. The fittings are a standard size, and the system uses a bubble flare method. This is similar to the double flare, but not as complete. 3/16" is the standard line size though. If You're looking to make repairs or replacements, the line can be picked up quite inexpensively. I recommend grabbing way more than You think You'll need -- it never hurts to have extra on hand. Considering You'll be able to grab 50 feet or so for around $30, there's no reason not to get extra.