Seriously considering purchasing a speed board but after looking at what is available am a little bit confused as to what size would suit me best.
If you look at *board isonic's for example sizes are all quite close together, so what is the best way of choosing the "right" size.

At 75 kilos and looking to use primarily my 5.8 and 6.6 sails would I go for the W49? Or are there other variables I'm not considering?

Depends somewhat on your skill level and how much you hope to use it. The smaller speed boards (ie around 50L volume) can be harder to sail and need stronger winds to get going - so you might get to use a small one less often. And you can be more likely to get skunked away from your launching point necessitating some swimming. I'm guessing the sails you mention are cammed? Maybe Koncepts judging by the sizes? You will want cammed sails for speed and a good speed fin. A speed board isn't so useful without a good speed fin.
If it is your first speed board a slightly larger size (ie 60L approx) will be easier to use and you can get a bit more TOW with it.
But on flat water on windy day on a very broad reach a speed board will give you a seriously fast ride and it will feel like there is no upper speed limit! Caution: This practice can be addictive!

You are right, KA Koncepts. From what you say I'm thinking along the right lines, about 60 litres.

So I'm guessing an approx 70 litre board is for bigger riders or using with bigger sails. This size seems so close to a small slalom board though.

it depends on where your going to sail, if you have to sail through chop to get to flat water you will need a slalom board , also take into consideration most dedicated speed boards don't jibe all that well
i would find a low volume slalom with a nice narrow tail, you mite be 1or 2 knots slower in a strait line but pull off a few nice planeing gibes and that can be as addictive as down the line speed

Thats the other option I have toyed with as well. Currently have a Manta 59 but am unsure what to drop to size wise without having a big overlap. Have also considered changing the 59 for a 54 as it would suit my weight as well but in marginal and gusty conditions the 59 is handy, would the size difference make that much difference, 86 down to 74 litres?

Your profile indicates you live in Bathurst. I've lived in a couple of inland places. Windsurfing there was frustrating compared to on the coast with gusty winds and short runs. Where do you go sailing and do you get winds that can power such small boards?

True speed boards have a much narrower tail which will allow for greater speed but with all the caveats stated above. A lot of the choice depends on where you are sailing. If you want something for regular trips to a true speed spot then go small but if you are looking at going faster where you normally sail then a smaller slalom might give you more fun.
I have had one outing on the JP 45 (53L) where I got some awesome speed and acceleration but spent most of the session swimming. Something like the Manta 49 might be a good addition to your 59 and will carry a 6.6 with the right fin.

I have moved from a JP FSW 91L (60 wide) to a JP slalom II 84L (55 wide)to a *brd Sonic 100 (58 wide) as my main GPSTC boards. I have had consistently faster speeds from the Sonic in general sailing (but have also changed sails etc and hopefully got better). I also have a 50 wide modified high wind slalom that I have set my peak PBs on but it only gets used infrequently.



I am just under 75kg and will be looking at something 55 ish wide as my next general board.
PM Snides8 as he had a Manta 54 and is around 75kg.

Mobydisc whilst I live in Bathurst I don't sail out here because as you said it gets too gusty. I only sail when we head over to the coast.

Lao Shi the Manta 49 does look good, but is it a speed board or a small slalom?

Manta 49 is a small slalom but capable of some good numbers.
gpsteamchallenge.com.au/sailor_session/show?date=2010-03-04&team=10
Post from Chris Adamson of Swan river mob

Looks good. Now to track one down.