As per NEMA VE 1, Metal Cable Tray Systems, cable tray typical lengths, not including connectors, are:
(a) 3m ± 5mm (10 ft ± 3/16 in.);
(b) 3.66m ± 5mm (12 ft ± 3/16 in.);
(c) 6m ± 9mm (20 ft ± 5/16 in.); and
(d) 7.32m ± 9mm (24 ft ± 5/16 in.).

Actually, we rarely have clients use 3.66m and 7.32m long cable basket trays. Very few clients would use 6m long basket trays. But the 6m length would cause many issues. Firstly, 6m long cable trays is difficult to do galvanizing. Most electrical galvanizing pools are not long enough to lay in very long cable trays. Secondly, they cannot be lifted by a forklift since manufacturing to warehouse stocking. Thirdly, they will be not flexible for transportation nor installation, as they are too heavy and too long.

Our Bonet wire mesh / basket cable tray use 3 meters as our standard length. Considering 40ft containers inside length is about 12.02m, and there would be gaps among two pieces of mesh cable trays, the actual cable tray length is controled at 2997mm (tolerance +0mm, -2mm). In this case, a 40ft container can load 4 lengths of cable trays. Final container loading situation would be like below picture shows.

Although 3m is our standard wire basket cable tray length, sometimes we will recommend clients to swift cable tray length from 3m to 2.9m. If not, the situation will be like below picture.

As we can see, there would be a lot of space waste, which makes unit freight cost much higher. In order to fill more cable trays into 20ft containers, we will recommend client to choose 2.9m as manufacture length. A 20ft container inside length is about 5.88m. Therefore using 2.9m length can load 2 lengths of wire cable trays in it.

Will 2.9m length cause any inconvenience? Except for increasing a little more connectors, they are almost the same comparing to 3m length. For example, if a project require 1000 meters of wire mesh cable trays, choosing 2.9m length will need 12 more places of connectors than choosing 3m length. It is a very small number in the whole project.

Besides 3m and 2.9m, we had made 1.2m, 1.5m, 1.8m, 2m, 2.2m and other lengths of wire basket cable trays. It all depends on your demands.