This is my own prefrence, but I prefer Rome than Burton. As for choosing what board and what board size, I go by looking for a board that fits my prefrences. How does it handle? What should it be used for? Etc ...
All of these questions can be answered by searching and looking for reviews. Although sometimes a board hasn't gotten a review that is beneficial or even been ridden, sometimes you have to take a chance and give it a go. For a first board, I would always go with something that has a lot of positive reviews so you know you're getting a board that you can learn on.
Picking new boards and experimenting with new boards and all that would be for someone that is somewhat experienced enough to know what to look for in a board and can tell the difference from variable X and Y.
When choosing board length, look at the board and see what weight/length ratio the board recommends. Always go and look at the board in person and see if it fits your reqiurements. These tips are something I do, it's not necessary but I like to feel insured that I'm buying something that fulfills my needs and wants before buying. It also helps if the place you're buying from has a spectacular return/exchange policy.