The OEM stock fin for the 2008 GO 144 was a 48 cm.
GO 144 144 246 79 50.0 11.0 5.0-10.0 Drake R13 Race SL 480 S-Flex 42-52 Deep Tuttle
If the above are the specs. for your GO 144 the 48 cm S-Flex would be a good place to start.
Are you sailing in lots of wind (i.e. > 15 knots most of the time)?
Your sail sizes would seem to be somewhat "limiting" for the lite wind potential of the GO 144.
For that board, around 7.5-8.5 m2 would be optimum in 12-15 knots (probably the optimum range
for the GO 144).
If you get larger rigs, then the fin range would be 42 cm-56 cm.
42 cm for your sails < 5.5 m2
46-48 for sails in the 6.5-7.5 m2 range.
52-56 cm for sails larger than 7.5 m2 in light winds.
Hope this helps,