Purely speculative theory, but one possible explanation for what happened to a lot of the Shardblades and the difference between Alethi blades and Szeth's is that the Alethi blades were reforged to create the extremely large Shardblades that are seen nowadays. We know that they can't create new Shardblades but Shardplate rebuilds itself when infused with enough Stormlight so who's to stay that in the days after the Radiants they didn't have a means of "growing" larger Shardblades by combining them together and infusing with Stormlight?
The larger blades that the Alethi have seem more adept at mowing down large numbers of foot soldiers, Dalinar describes fighting with a Shardblade as being very different than using a normal sword; more large arcing swings than precise. This sounds like an adaptation made over the years from the Shardblades being used to fight in Desolations to being used to fight battles against human armies. And because only other Shardblades are really a threat against someone in Shardplate this fighting style doesn't seem to lend itself to self protection.
From Dalinar's vision, Shardblades don't always just pass right through creatures, when fighting the Midnight Essences:
Moving fluidly, Plate clinking, the man struck at the beasts. He effortlessly sheared a monster in half, flinging pieces into the night that trailed black smoke.
While it may just be he killed it then the blade continued on to cut the thing in half it sounds more like the Shardblade had a physical effect on the creature that they don't have on people or normal animals. When fighting something like that you are going to need to fight with the Shardblade differently than in their large scale battles.
Since Szeth's blade comes from the Shin who are more pacifist they would of had no need for the "adaptation" so his blade is still the normal length.