Start small!

Our first RV was an 18 foot Jayco. Two bunks, a small kitchen, bathroom with small (think small elf infant!) bathtub and shower, and a living/dining area - the kitchen table folded down, the seat cushions spread out and VIOLA - our bed! We were quite smitten with our little travel home and thought we were quite bad ass! Yup, we had given up the tent and gone over to the dark side...a very small dark side, but it was all ours!
At this point in our lives we are a family of four (down from seven!) but even so, doing anything in this RV was a bit of a dance. If I was in the kitchen area cooking, I had to stop and move so the kids could get out of their bunks and get into the bathroom. If the kids woke up early and we wanted to sleep in, we ended up with kids eating at the table aka our bed.
But it was a good first time RV. It allowed us to experience the RV lifestyle without having to make a big commitment. And it allowed us to learn what worked and what didn't on a smaller scale.
Having a smaller RV allowed the hubby to get used to pulling a trailer and learn all the ins and outs of how to hook it up to the truck (a jeep at the time!) and how to back it up and park it in a site.
We also were able to make the transition from tent to RV'ing in an easy manner - it wasn't such a huge shock and truly felt decadent.
Until we had a long weekend trip where it rained most of the time. And we were on top of each other for three days. Three. Days. Three. Long. Days.
Yup, we had learned how to tow a trailer, back it up, park it, pack it. Now we realized we needed something bigger so that we could all enjoy our weekend getaways without feeling like we were sitting on each others laps all weekend.
Hence the RV we have now - all 37 glorious feet of it! So go ahead and get that smaller RV. You will learn invaluable lessons that will serve you well when you upgrade, and upgrade you will for one reason or another. You may want more room for the kids to be able to bring friends, or you will want more room so that you can bring other family members, or you may even want to sell all your stuff and commit to a full time RV lifestyle. But whatever the reasons, you will eventually upgrade to a bigger RV and when you do, all the things you learned with your first, small, cozy RV will go with you to your bigger RV.  

Fair winds!

Jaye