Three points:
1. As everyone here has said, there is no canon design for the Titan, so we are all free to speculate what kind of ship it is.
2. The impression I always got from Nemesis is that Riker's command was the biggest, baddest new ship in the fleet. I mean, isn't that why it was named the Titan? (for the uninitiated, the Titans were a group of extremely powerful gods in Greek mythology). So what does Pocket Books do? Make the ship small, and take the name out of context (of Nemesis, anyway) by establishing that the name was in fact meant to represent one of Saturn's moons.
3. As for Riker passing up command after command: To this day I have always felt that it would have been a better move for TNG to have had an eighth season in which everyone's personal stories are wrapped up and they go their separate ways. Picard and Crusher would get married and retire, Riker would get married to Trio and finally take command of the Enterprise that he always wanted, Worf would return to the Klingon homeworld, and Data would get command of his own ship, bringing Geordi along to be his XO. But instead they went the movie route because that was more money in their pockets, and the result was four mediocre films with Kirk's silly death, the loss of our favorite ship by incredibly silly means just to have it discarded for some new soulless replacement, the total suspension of disbelief of this same crew serving together for 9 more years, the stupidity of the Borg Queen, Patrick Stewart increasingly playing himself rather than Jean-Luc Picard, and the overall theme of the "Picard and Data Show" to the detriment of the other cast, with the final denouement of having Data die an incredibly silly death just to leave his idiot brother singing silly songs.
