Get Double Butted DT spokes, will be a few quid more, but a better wheel.
559 is just metric for saying 26" and has nothing to do with the place where the spoke nipple are in the rim.
17.5 is the internal width of the rim, often approximate.
Offset is for rims where the spoke holes are set to one side. Normally omitted unless you know you need to use it.
SJS give the ERD as 533mm as mentioned above
(have a read of the how to measure
for some info)
double checking info here http://www.hopetech.com/webtop/modules/ ... nsions.pdf
for hub dimensions you can see the Bulb and Pro2 (135) have near as anyone cares for spokes the same measurments
Complete wheel data
Hub Hope Bulb 135mm
Right flange ø: 52
Left flange ø: 56
Centre to right flange: 20.5
Centre to left flange: 31.5
Spoke hole ø: 2.6
Rim DRC MT19
Effective rim ø: 533
Offset spoke bed: 0
Spoke length for 32 spokes
Crosses Right Left Tension L/R
0 240.1 239.3 65%
1 242.2 241.6 65%
2 248.3 248.2 65%
3 257.2 257.7 65%
4 267.2 268.5 65%
Based on 12mm nipples.
(to back up measurements above)
probably get away with 256mm with 14mm DT nipples or 258 both side with 12mm DT nipples. As 257 are often not stocked.
But leave it to the bike shop, for 20 odd quid it's worth it and they have the lengths to try with.
260mm would be too long so you couldn't use them anyway.
It's well worth measuring things yourself as plucking them off a random website for ERD and there are many Bulb and Pro 2 types of hub around (or at least measurements that differ for them)