Reading and looking at bluefish pics online, you can use this as a chart and kind of figure it out if in between.
15 inch bluefish is around 1 1/2 pounder.
20 inch bluefish is just over 3 pounds.
25 inch bluefish is just under 6 pounds.
So that gives a good guess on what they weigh. Weight can vary obviously so if yours looks quite fat for that size, then I would guess to the high side.