Wii Fit Weight Limit
Although the balance board is able to withstand much greater weights of around 600 pounds (because you would exert different amounts of pressure by jumping around on it), there is a weight limit that it can accurately measure.
The weight limit for the US and european models is set to 330 lbs (149.6 kg). The japanese Wii Fit, however, has a weight limit of 300 lbs (136 kg).
There seems to be a small trick to increase the weight limit of Wii Fit by 10-20 pounds (4.5-9 kg) though: apparently, if you would use it on a thick carpet (or a blanket folded several times), it tricks the weight sensors and let them think that you are slightly lighter than you are. You could then use a temporary Mii until your weight gets under the weight limit so you can use Wii Fit without the carpet 