im looking at getting a fixed crank for my bike, all i know is it is a lotus bike roughly in the early 90s i think. it looks like the pic i provided.
now what im trying to do is figure out how to measure for the correct sized BB so i can get the right fixed crank for my bike or if i can use my old one. the crank size i want is a 165-170mm with either 44t or 46t if that helps. any help would be appreciated.
thanks!

BB spindle length is dependent on the crank you choose.

pick a crank first, then find the appropriate BB.

I don't understand the question - the crank isn't what makes a bike fixed. You need a fixed rear wheel, and then to work out the other details.

Seriously, don't convert one of those lotus touring frames. This isn't about the frame being classic or nice (though it is both things). The frame is a beast. The pictured bike is my old roommate's and it is still in my living room (and it is for sale too). Without any of the gear loading down the bike, the thing is impossibly heavy and most of the weight is probably the frame. Find something lighter and with less relaxed gemeotry.

And I think that bike is older then the 90s.

the reason i ask the BB size is cause im trying to pick the right crank, on bens cycles website i want to get a crank from them since i have a return im sending back, but some cranks say to use with a BB size of xxx mm Thus me asking how to measure my stock one for reference. cause i want to make sure i buy/order the correct BB for the frame.

the frame is amazing, i love riding it as is. ill be keeping all the parts in the garage if i ever want to swap it out of fixed. to me im more than happy with the weight of the frame im sure if i rode something lighter i would be all over getting a different frame, but i have this one and not many do so it makes it unique and im not going to paint this frame everything about the frame will stay the same color and stickers so forth, just updating the rest.

i have the wheels and rear wheel/hub/cog and locking ring, my name shows noob but i've done reading on this site and im a member of many other sites so i know to read before asking.
thanks

and thanks to the others for info as well.

You aren't listening. The crank you choose will SPECIFY which bb to use (and length) to attain a certain chainline.

You do not pick the bb and then look for cranks (well you could, but the results won't be spectacular).

actually i am listening, but the more i ask the better results i guess, i dont want to order the wrong BB for my frame.
and i know they recommend a bb. just want to get the right one.
continue.

There is nothing for you to measure. Get the bb they recommend for the crank you are buying and make sure you choose a 68mm English one and not a 70mm Italian one.