Start from what already fits. If you run GMA pallets, 48 x 40 in drops into the racking you have. If a customer ships parts to you in 48 x 45 in containers, match them so your dunnage and rack slots work with theirs.
Then the part. Long parts set their own footprint: 56, 64, 70 or 78 in long on a 48 in width. Small dense parts go the other way, to 32 x 30 in, so a full container stays under its load rating.
Height last. Taller holds more, but stacks fewer high on a truck and needs more clear height in the rack. Every height is listed on its footprint's page.
Choose "Other / not sure" and tell us in the message what the container has to fit: the rack bay, the line station, the pallet you use now. We work out the footprint with you before we price anything.
The reply names the container we would quote, with its collapsed height, return ratio and trailer count written out, and a price for your quantity and ZIP.