Hello and welcome to our community! Is this your first visit?
Is this for diving caves in the UK or abroad?
For the UK, as the sumps have generally bad vis, are smaller and you generally stay very close to the line, a shorter reel with thicker line is best. I think my UK search reel has about 10m of 3mm cord on it which is bungied to my arm. I generally have another larger reel with me as well in case I need to fix or lay line, which generally has around 50m of 4 or 6mm polyprop on it, which I clip to one of my back D-rings.
For big french sumps, I have a larger safety reel, with around 30m of 1mm line on it, which lives in my drysuit pocket. I also usually take a couple of smaller reels with 10m of 2mm line on them for jumps or line fixing if needed.
Its for the UK but not sumps I will be more than happy in the mines, dont fancy that CDG stuff TBH. I have about 40m on an existing spool so think that will be OK for jumps and also a primary reel. I am thinking of France next year (Lot) so might sort a different safety for that trip.