First, you asked about class size... personally I really dig the fact our class is big, it gives you room to be yourself and find plenty of unique people to spend your time with. It also means setting up study groups with someone with your same study style isn't as hard, and we have more people contributing to tutornet, our online note sharing service. Also means more resources allocated to the school in general. Also I feel like gossip in our class is a little more restrained compared to some schools I have acquaintances in, not that that's really a big deal.
Second, ISP (you got the name right) isn't something I know much about since I didn't choose that pathway, but I'll share what I know. Basically you go through Anatomy like everyone else and at the end of Anatomy you can choose to be ISP. Basically you give up attending lecture each day in exchange for "Modules" or a variety of texts that take you through the curriculum. You are free to study any time (ISP students have their own library with 24 hour access) and take tests on your own schedule (within reason, you still have to take Step I of the boards the same time as everyone else) and show up for CAPS class once a week (which is basically a clinical skills class everyone takes).
None of my friends in ISP have complained about a lack of direction, you still have Module leaders who are professors who can provide guidance and answer questions. Some people I know HAVE complained about feeling isolated, ISP tends to attract people who don't show up at campus very often or have families, thus you don't have the regular social contact that you get with lecture. That's honestly a big deal for me, and the main reason I chose not to do ISP. The friends I know in ISP, however, are for the most part happy with their choice, more than happy to not show up on campus very much, have more flexibility etc etc. I think the phrase I hear most often is "done with that classroom bullsh*t."
About 25-60 students choose the ISP pathway, so know that its an option, not a requirement. And remember, I didn't do it, so my take on it is not definitive.