Physical power as strange as it may seem to you, is not so important in our line of work. If you correct your body posture and you apply the proper "body mechanics", you will easily find an effective way to protect your hands and back on the one hand, and of course you will definitely add more years to your career.
Of course some clients are much bigger than some others, and it makes no sense to try to work on all of them. Trying to perform some deeper work on someone who is twice your size, perhaps won't have the desired results for the client and can possibly put a lot of stress on your own body. In these cases there are some factors which you should have in mind, in order to find out what your limits are. Yes, everyone should know his limits
The first one is experience; yes with experience
you will learn how to take advantage of your own body weight and apply force more effectively, without injuring your joints or putting unnecessary stress on your muscles. Your physical strength
will increase anyway as time goes by, either you want it or not (of course if you don't get injured on the way). Using plastic or wooden tools to assist you while applying force on someone's body, isn't a solution. Using cold and hard tools is something that I never encourage you to do; therefore, according to my opinion, these are not part of any solution. On the contrary, they can easily cause harm.
Another factor is compatibility
. What I mean by this is: "to know when you have reached your limits". When you first start your practice, you are allowed to be a little picky when it comes to body types. You won't take in a customer who is twice your size, instead you will choose people who are more close to your own figure. These people you can handle more easy. As months go by, you will develop an awareness and it will be much easier for you to decide who you can handle, and who you should avoid. Very soon you will discover that working on much bigger people than yourself, isn't a problem any more. Therefore, you can keep on experimenting until you reach your limits, and come to a point where you'll just say "this is the furthest I can go". If you push yourself above these limits, you will know that you put your personal health in danger.
It's not a good idea though (marketing-wise) to turn down clients. If you work alone and there's no other therapist to take your place (with greater physical powers), you can avoid misunderstandings by just being honest with your potential guests, and inform them about you "limitations". You will be amazed to find out that many people who seek massage therapy are not the "hard core" demanding ones. Even bigger folks can feel rejuvenated with a milder approach. Therefore don't make conclusions by yourself, and every time you see a tall person or an athlete, don't take for granted that "he's here for the deeper work". You should know that deep tissue massage isn't everyone's preference.