Speaking as a complete beginner to bicycle engineering who just converted two bikes (one fixie, one single speed). Assemble it with what you've got and measure the chainline as per Sheldon Brown.

Then either sit and look at it and decide if you can reasonably shuffle/space things about to bring it into alignment or buy a new BB of the appropriate length.

This may not be by the book but old fashioned "trial and error" worked for me.

On the fixie, I guessed the BB size because it originally had a cotter pin one. It was a good guess because the chainline was only 2mm out. I put some washers between the chainring and the spider which took it to under 0.5mm difference. I consider that to be close enough.

On the single speed, I was doing a very rough, cheap-arse conversion on an old racer bike using the original crankset and replacing the rear gear cluster with a screw-on freewheel. The BB was worn out anyway so I knew I was going to need to replace it. I assembled it with the old one and found the chainline was about 4mm out. Bought the new BB 4mm shorter, measured again and found it was 0.75mm out. Again, close enough for government work.