My advice is to buy macro lenses USED. Why? It is THE category for buy, use for a week or a month, and then put away, only to sell back to a dealer three years later. Macro lenses of up to 20,30 years of age are often seen in dealer cases looking BRAND-NEW! Macro lenses are tools that many buy, then just never end up using!
As to length: Tamron 90mm AF-SP; Tokina ATX-PRO 100mm f/2.8. Both have nice bokeh, good lenses, I'v owned a Tamron 90 for 15 years or so. Tokina-prety imager. Canon EF 100MM Internal Focus f/2.8, owned it for half a decade, UGLY, sharp-sided diaphragm gives rough OOF highlights, but is inexpensive used. Sigma 150mm macro: beautiful imager, just beautiful. Sigma 105/2.8 macro: bitingly sharp lens, lens rendering color matches Canons quite well, looks yellow on a Nikon.
I really think a 90,100, or 150 or 180mm macro is the most useful. I had the 180 EX HSM 3.5 Sigma macro...really,realllllllly nice to be able to get true life-size 1:1 at 18 inches from the back of the camera!!!
Short macros like 60mm and 50mm...fine for 8x11 inch documents or paintings, and good for plants, and scenes outdoors, but not long enough for small objects.