So...I got a 149 cm snowboard for free. I wear a size 10 boot and am 5' 9". Is it too small for me?

That seems alright, it should be just fine. You can put the boot in the bindings and see if there is any toe overhang but you should be all good with just a size 10

Yep II agree w/ Steve 100% in regards to sizing. Also keep in mind that with your height and weight, you will have limited speed for descents. I know it's common for riders to use smaller boards for jibbing but why limit yourself? RIDE EVERYTHING! I would suggest maybe post on your homies, local craigslist and or forums to see if you may be able to sell or trade someone decks. Or just be a sweetie and give your sweetie yours and buy one that fits you. hehe. Hope this helps ;)

First off PD didn't not come close to being right. At 5'9" the normal board would be around a 155. Ranged normally between a 154-160(but its Personal choice). Also a smaller board goes slower then a longer board but it is easier to control. Lastly the only factor on if you need a wide board or not is your boot size compared to the width of your board. Longer boards are naturally wider so if you have a long board you might not need a wide board. Secondly since the board is smaller then your ideal size you might have overhang. That depends on your boots and board. If your board is not a wide and a normal you might have overhang. The only way to tell for sure is to put the boot into the binding on the board and look at it. Hell if you can do that but are still not sure take a picture and post that and ask again.

At 5'9", you need a board between 158 and 164. You can always ride a smaller board however. It will go faster, but will be easier to control. You need a wide board if your boots are larger than size 11, so the width should be fine. Overall, although the board is not perfect for you, you will be fine.