The fins that are supplied with most Starboards are pretty much "in the middle" as far as the fin range
for that board goes.
So, if a 48 cm fin came with your husbands Futura 133, the true fin range is probably 4-6 cm on either side of 48 cm.
So, 52-54 cm would be great for light winds and large sails (like 8.5 m2) and conversely 42 cm would not be too small for a 6.5 m2 rig.
So, yes, a 42 cm should work, but I suspect that you need to do a little more learning/training on fin control.
My guess is that you are probably a lightweight (most girls are) so try to find a 42 cm fin and
also try to learn better control.
If the board is "turning over" when you try to get into the footstraps, my guess would be that you are not getting your back foot place correctly over the centerline just ahead of the rear straps while you get into the front strap.
That back foot is all you have to "steer and control" your board, so it has to be placed correctly to give you that control.
Hope this helps,