Hi vinayak very honestly you will need to do self research on this, I mean only you know your lifestyle and more. There are many questions you will need to answer yourself. Like in your absence who will be with the dog, will he be alone for major time of the day or is somebody going to be with your dog to feed, walk, etc.. So please do an extensive research on this, you will need to understand the breeds temperament its behavior patter exercise needs etc..
As for boxers are concerned they are not meant to be kept indoors I mean in a one bkh. They need space to jump run and move around they are high energy level dogs. Mastiffs on the other hand are okay with that much space but still not recommended space limit as while walking around they might damage or break things kept on tables, side tables.
My suggestion to you would be read about every breed you like and then decide which suits your lifestyle and also keep in mind vet bills as some breeds are prone to health problems.