As per this site
https://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx,
a 235/55R17 tyre is 690.3mm diameter and 2,168.64mm circumference, a 215/60R17 is 689.8 / 2,167.07mm whereas a 255/45R18 is 686.7mm dia and 2,157.33mm circumference.
From what I see, with a difference in diameter of less than 4mm it will be like having a slightly less-worn tyre as your replacement, since tyre tread normally starts out at 7-8mm depth and drops to 3-4mm before many people change them (remembering that this difference should be doubled as is on each side of the tyre, so tyre diameter drops due to wear by about 10-12mm during its life).
Therefore I would think a 17" spare with either a 215/60R17 or a 235/55R17 tyre should be fine. I would think probably best with the 235 to keep closer characteristics with the remaining 3 tyres.
But I am not a tyre expert and woud be very happy to be corrected/educated by some of the more experienced fellow forum members....