Our first cruise was 11 years ago for our 40th anniversary. We didn't stay in North America but did remain in the hemisphere. It was a 2 week Norwegian cruise from Buenos Aires along the coast of Argentina, around Cape Horn and up the coast of Chile, ending in Valparaiso. Many interesting ports and sights along the way. Weather ranged from quite warm at the beginning and end to pretty nippy at Cape Horn. The length was about max for a first cruise in our opinion. The ship was very nice with, in retrospect, more amenities than we needed (example: a casino). We have, however, since discovered that we're happier with smaller ships (think Viking Ocean at 1000 passengers vs 3000+ on Norwegian or similar). One advantage of a smaller ship is that they generally tie up at a pier no matter where you go. Contrast that with a really big ship that often anchors out such that you have to go ashore in launches. (Too many flashbacks to going on liberty in the Navy via a motor whale boat.) Many other advantages including nobody under 18, no water slides or climbing walls, more subdued (but still excellent) entertainment, etc.
We've also done an Alaska Inside Passage cruise (about a week) on an Alaskan Dream ship. They have smaller ships 80-130 or so passengers, IIRC. In my opinion, that's the only way to do Alaska because the big ships can't get in as close to glaciers as the little guys. Also, they have more schedule flexibility than the big ships. That means that if there's something particularly interesting going on (a pod of whales appears, a glacier is calving heavily, etc.) the captain can make a split decision to linger rather than to move on.
Those are a few thoughts off the top of my head. Feel free to PM me if you have any specific questions.