i have a mid star 40 and am converting it to seaplane it is 6 lbs what size electric motor will i need

Is that the all up weight? Does it currently have an engine on it? Seems a bit heavy as Sig suggests 5# for AUW. However at 6# you're going to need 100W/Lb. at least if not a tad more for floats, floats create more drag than wheels. If it were my plane I would consider at least 700-800 W in and considering the efficiency of the motor just how much actual power you will have to produce thrust. You can always reduce the throttle to a comfortable cruise speed.
You may want to check out Inno8tive Products Innov8tive Designs/Scorpion products
They provide battery/ prop charts for their motors which will give you an idea of how much power and thrust each motor produces. These are not estimated but actual tests.
I would consider a 4S pack as well. Consider how much prop clearance you will have with the floats.
Hope someone who has actually flown a Mid Star on floats will chime in.
Out of curiosity, what floats are you using?

If it was built that heavy plus the weight of the floats plus flying off of water, I would install a Power 46 , a 60amp ESC and run it on a 5 cell 3800mah pack.

i have built the floats with plane and floats it is 6lbs no motor or esc i was flying it with thunder tiger 42 with wheels