You can certainly use a smaller rig in your Futura 141.
The sail range is just an "optimum" sail size range.
You can go larger, but the board width will not support the early planing abilities of the
You can go smaller, but the width of the board then starts to become a liability as too wide
a board may not handle the sea surface conditions (chop etc.) as well as a smaller/narrower/
less volume board that has the 5.7 m2 sail within it's optimum sail size range.
So, give it a try, and if you do not like the way your Futura 141 handles, perhaps rent/borrow a
smaller Futura (101 or 111) that has the 5.7 near it's sweet spot for both windspeed and sail size.
Remember to try a smaller fin.
Too much fin can make your Futura 141 very hard to sail in 17-22 knots windspeed (where you would need a 5.7 m2 rig).
If you have never experienced true "tail walking" you might get that experience if you are well powered on your 5.7/Futura 141 combo with the stock fin.
The absolute loss of control when you have way too much fin and your board tailwalks is something every windsurfer needs to experience at least once.
Then you will change fins more often than sail sizes.
I weigh about the same, and don't really get on a >6.0 m2 rig until 18-20 knots, and the Futura 101 handles 6.5-5.5 m2 rigs very nicely.