Hi, I learnt to board using a 3m Radsail Pro on a beach in clean winds, which did me just fine (I'm 14.5 stone) ad got me moving well. The Pro is meant to be of similar flying characteristics to the Blade (def. not in material durability though!). I doubt that your Blade at the size it is will give you any nasty surprises or major challenges and the extra lift it does give you will probably help you stay up on your board rather than hinder you. Bigger kites are slower through the air and hence give you more time to think etc, but if the wind kicks up when your in the middle of a session, can be scary! Personally, I have preferred a kite that I needed to work a little bit to get me moving.
After about a year of regular practice in increasingly stronger winds I purchased a 6.5m Blade II and 10 and 13m Guerilla II Arcs, which I still love to use. Obviously newer kites have more advanced design features, but I don't think they are that far removed from earlier designs and if you buy a decent make, would go second hand for future kites until you are really proficient at boarding. Depower is fantastic and will bring another dimension to you kiting, and is definitely the way to go after a while, but there is also something special about boarding with a FB as well!
Have fun!!!