While no official set size was ever announced, Crosstrainer was to include all 151 Generation IPokémon (Bulbasaur to Mew), as well a number of Trainer and Energy cards. It is estimated that the set would have included around 170 cards.
The official release date for Crosstrainer in North America was 10th July 2001; however, it was never released. It would have been the 10th English Expansion.
Because Crosstrainer was devised by Wizards, it was not going to be released in Japan.
Crosstrainer went through a number of name changes since it was first announced. Those name changes included Pokémon X, Tournament Set, and Base Set 3.
Crosstrainer was intended to be a new Base Set revision, building upon the standards set by Base Set 2. It was said to include cards from Base Set 2 (which was Base Set & Jungle), Fossil, Team Rocket, as well as introduce 30 "new" cards that were previously exclusive to Japan. It was thought they may have been the Vending Machine cards. Other rumors had said that Crosstrainer was going to include cards from Gym Heroes/Challenge, but it was very unlikely.
Crosstrainer was ultimately axed from Wizards' product line up in the summer of 2001. Wizards representatives said that the company wanted to instead focus on releasing the Southern Islands promotional set and Neo Revelation. The rumor that Nintendo actually didn't want Wizards to release a new Base Set revision at that time still persists, and actually seems very likely.
Although Crosstrainer was never released, the concept lived on in Legendary Collection, which was a Base Set revision like Crosstrainer. More so, the idea of adding cards from Vending was resurrected for Jamboree, another unreleased set.