There's no definitive answer to your question, because it depends:
- How often do you expect to need to bridge or un-bridge these?
- What is the accessibility of the area like? Are other components nearby making it difficult to work on the bridges?
- Who are these jumpers intended for? End users? Hobbyists? Engineers?
- Are you space-constrained? The bridges could be 0201 component size and require magnification to work on.
- I expect this to not apply, but what is the voltage? Higher voltages would require a larger gap.
Have you hand-soldered 0402 components? The gap between pads of an 0402 is about 0.5 mm. On the larger end, 2.54 mm (0.1 inch) is the same distance as typical perfboard (proto board) and those are relatively easy to bridge with enough solder (of course the copper-to-copper pad spacing is probably closer to 1 mm). As long as you keep the gap wide enough for the fabricator tolerances (e.g. 0.15 mm or 6 mil is sometimes a minimum), and voltage is not a concern, select a gap that meets your criteria, similar to the ideas I listed.
As for track width for closed-by-default jumpers, that depends primarily on current and how much effort you want to go through to cut the track later. If these are just short tracks for logic voltages, consider using an 0402 component with a 0.2-0.3 mm wide track connecting them. That's what I do for prototypes and it works well. Mind you, cutting tracks with a knife later is a bit messy. I prefer to use 0-ohm 0402 jumpers instead of tracks meant for cutting.