In fossil fighters where is the strongest fossil before rossie gets kidnapped

Depends on your opinion of "strong." Vivosaurs get stronger every time they rank up. They can rank up through a lot of fossil battles, or by you getting a good score when cleaning parts. Every 50 points (the total... so this includes cleaning points AND battle points) gives them 1 additional level (50 points=Rank 1, 150 points=Rank 3, etc.)
The different sizes also play an important part. You can have teams of only one size, or a mix of the sizes.
Small-sized vivosaurs aren't #1 for attack power, but they're great for support effects. Having Tro and M-Raptor in the S-Z give a great boost to your A-Z vivosaur. Of all the small-sized vivosaurs I've found and used, Nychus has the strongest attack. I believe it's 64 when Nychus is rank 12. Small-sized vivosaurs typically have low-FP skills, so they're good towards the beginning of the game for doing as much damage as possible.
Large-sized vivosaurs don't have the best support effects of all times, but they have great attack power. Having a team of large-sized vivosaurs is great for taking out enemies quickly. The large-sized vivosaurs don't come with low-FP skills, so it'll probably be easier to use them when you get a higher fighter level. Having a vivosaur on the team with FP-Plus helps, too.
Medium-sized vivosaurs are more towards the middle. They don't have the highest attack, and they don't have awesome support effects. They're meant to be balanced. I can't say I've done a whole lot of battling with them, but they're good additions to a team.
Personally, I haven't used titanic-sized vivosaurs at all. Ever. So I honestly don't know what kind of attack power they have. They're long-range class, so they'll do the same damage from the S-Z as from the A-Z, but their support effects will punish your A-Z vivosaur. Their skills also cost quite a bit of FP, making it hard to use your full team very well. There are 3 exceptions to the bad support-effects rule for titanic-sized vivosaurs, but none of them are long-range. The 3 together still make a very powerful team. You have to do a few "quests" of sorts to get them, though.
Personally, I don't think size really matters with your teams. Just making use of type advanatages/disadvantages and ranking up your team will get you through pretty much anything and everything.