Just retrieved my board from the gf's and am wondering what type of fin I need.

Looks like a (bic/tiga) Trim box -

It's a Fanatic Bee of some sort if that helps.

Am I right in assuming that I've lost the bolt thing to hold it in?

Yes. I agree with Davmen. Not Tuttle and not Powerbox, and looks like Trimbox to me also. The bolt that holds it in is a standard stainless steel 6mm metric bolt like those used in Tuttle and Powerbox. You just need to find the right length to suit your board and fin.

What would you recommend for a 140ish litre board about 290cm long.

Also are there any places in melbourne where I can get trim box fins, was hoping to get out this weekend but that may not be possible anymore.

my BEE 124 is a power box, but that is definitely a trimm box - flat top, hinge in front and screw in back
fin size is based more on sail size and sail size on board size
that board probably handles 6-9 m meter sails
6 - 32-36 cm fin
7 - 40-44 cm
8 - 48 cm
9 - 52-56 cm
there should still be trimm fins about
sold all mine -> due to foil , slalom boards - all fin boxes seem to be going towards tuttle now

Definitely a trim box. Recognize the board. 137 litre Fanatic Bee. Used to be bright green before a mate of mine went mad with the spray paint. The two plastic blocks slide in the track so that you can position the fin forward or back. Originally there were 3 but a rock at Stanley removed one. Had a lot of fun with that board. Good for light wind and learning, only problem I had was that you couldn't push it too hard in the gybe or else it would go sideways.

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Mmeyer said..
What would you recommend for a 140ish litre board about 290cm long.

Also are there any places in melbourne where I can get trim box fins, was hoping to get out this weekend but that may not be possible anymore.

I would try the shops first. SHQ and ZU. You may find one in the seabreeze Buy and Sell too.

www.seabreeze.com.au/Classifieds/Windsurfing-Accessories/~vedyw/Ht-Race-12.aspx?_page=1&search=gvyj2BKrnvnoSv1T0pAJr2rwTMMvsUzj

www.seabreeze.com.au/Classifieds/Windsurfing-Accessories/~vedy5/Bic-Freeride-31.aspx?_page=1&search=gvyj2BKrnvnoSv1T0pAJr2rwTMMvsUzj

www.seabreeze.com.au/Classifieds/Windsurfing-Accessories/~v33d1/Slalom-Up-29.aspx?_page=1&search=gvyj2BKrnvnoSv1T0pAJr2rwTMMvsUzj

The smallest trimbox fin SHQ has is a 42cm, is this too big for my setup given i'll be mostly using the 5.5m sail?

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Mmeyer said..
The smallest trimbox fin SHQ has is a 42cm, is this too big for my setup given i'll be mostly using the 5.5m sail?

Well that's it then. Your windsurfing comeback is over before it restarted!
Oh, wait, you could always cut 10cm off that fin with a hacksaw and it would be about right!