An easy way to get your wife to start working out and lose weight without insulting her is to participate with her. Make a date of evening walks. If you get a gym membership be sure to get one for both of you. She needs to know you care about more than her physical appearance so be sure to mention the health aspects. Change your eating habits a bit as well. Your support will make a big difference but she also has to do this for herself.
Start with a health check up for both of u, then whatever is the results start working on it, they would be able to tell u the desired weight for both of u guys age, then they will tell u to change ur diet & exercise regularly. Health is wealth!!!
Tell her you want the both of you to get healthier. Say you wanna diet and start getting in shape and ask her to do it with you. If you just tell her to do it by herself it'll make her feel bad and probably won't be successful.