Hi, I have a Kona One which I have been sailing for a couple of years. I have also sailed a couple of SUP's.
2 years is my total sailing time. I can gybe, but not on the plane as yet. I can water start as of my last session.
I have my eye on a Lorch Glider Whiteline 120L, and am not sure if I am ready for such a small board?
From what reports I have read about it, it should be a good board to gybe and for choppy conditions.
But is 120L the ideal size for me to step down to? I was thinking more like 150L?
I am 83Kg. Any help appreciated.
Hi Kenny, congratulations on your progress!
It is good that you are mindful about the volume, some windsurfers make that step too soon and it causes a lot of frustration, although I understand the appeal of small boards and the adrenaline associated with it! 
My personal opinion also concurs with your gut-feel: a 150-litre board would be a more natural transition, and will allow you to progress quicker in the mid-term.
All the best, and keep up the good job!
If you know water start, probably for your weight, 130 is better, i jumped from 165 to a narrow 110. I have been struggling for tacking for few months.now i am good with my 110. I am 58kg.
If you buy 120, be patience for the first few months, and you will be fine,..
As a longboarder ...
At 100 kilos my first shortboard was 160 litres / 79 cm wide
For 83 kg, using easy math 0.83 * 160 = 132 litres
and as you may have realized from current trends , width is your friend
the Fanatic Gecko 133 is 78 cm wide with a short 42 cm fin
that sounds like a great place to start and should sell easily if you get tired of it
having said that ...
local KONA ONE sailor, Sailboarder, does a bigger jump in size
he sees NO NEED to use a "big freeride"
believe his is more around the 110 litre size
and that - as discussed - takes MUCH more adjustment
(and he does NOT use it often)
with a Gecko 133 you will be tempted to use it more often ...
Width is as important as volume. It will provide stability and early planning. I'd look for something between 120 to 135 litres with width over 75 cm. The lorch is only 65cm wide.
Thanks Guys,
That's pretty much what I figured. 
But needed some confirmation. 
Cheers,
Ken
If I was you, I would go for the 120 L board. It's got plenty of width for early planing, and volume is completely wasted as long as you have enough to uphaul successfully. I can't imagine what it's like to jibe the long board and try to keep on the plane. Must require a huge radius curve.
But whatever you do, if you can tack the long board, learn to do the same on the short board.
If you can water-start and plane you can short-board. And you will enjoy it much more.
Stability is only an issue if you aren't planing.
