Choosing a bottom bracket

[IDB1]

Is the length dictated by the cranks?

What are the ramifications of putting a longer bb on a bike that doesn't need it?

Are other measurements (diameter?) standardised?

Looks like everything I have here is square taper but, since I will be changing cranks at some point, is there a better option?

As I replace them I was thinking of going for Shimano, what other types are there (and are they better)?

 

[barry2017]

If the spindle is shorter, the cranks might touch the frame when attached.

If it is too long, the cranks will sit too far away from the chain, and give you funny chainline, which could mean more derailing chain and less efficient drive.

Depends on your crank.

 

[FluffyChicken]

Also if it's too long you may have trouble getting it to shift to the top ring. My M970 (?) mech will only just do a chainline of ~53 and it's designed for a chainline of 50mm
Retro Chainlines are 47.5 as standard though you have the range of 47.5 to 50 to play with.
(MTB)

Though since we are in off topic road chainlines are different, so are tandems etc..

Hit old Sheldon Bown site and have a read.
Search 'Sheldon Bottom' will get you there quick in google.

The 'shell' change in length and diameter, though most are standardised around English in either 68 or 73.

Of course there are many other designs and an increasing amount in these here modern times all utterly different and have their ups and down.

Yes there are better than Shimano if you mean weigh less and cost more.