Mike,
You are going to need gear based on the wind here in UT. In till you are a expert level kiter boarder length isn't going to effect your progression. On a 142 with a 12m you are not going to get to kite often here. Most of us ride, big kites and bigger boards, 18m-14m kites are the norm most of the year, we do get days that we fly smaller kites but not often. I ride 12m more then most folks but with a light wind style surfboard from North kiteboarding called the nugget, I also have been riding lighter wind for a while so I have experance to rely on to help.
For now you want a twin tip that goes up wind well, plans easly, and has nice comfy straps and pads. There are used boards for sale every where, and most local kiters try and buy locally on this forum before looking out side the community, to help support all our habit!
I would recommend a medium flex free ride twin tip, with a bigger kite like 17m-14m you could get by with a 132-150cm board, Even if you were the fastest progressing kiteboarder ever it will be a year before you have the skills to want to taylor your gear to your style , weight etc.. If you want to ride with your 12m here you will need a huge board like the North spike, flysurfer door, sling shot glide, or as many of us do a big directional. Surfboard, raceboard, skimboard etc...
I weigh 175 ish, for twintips I ride the North select 135 for freeriding, and the 139 Gambler for boots. I could just as easly ride any size in the line but I had to pick some thing. We are not behind a boat at the same speed so being able to change power means you can make any board work.
You can go to Norths site,
http://www.northkites.com/products/boards/twin-tips/ I use the North line as a base becuase their website is great and every company uses similar "style" style boards.