How do I set the dpi only when my laptop is connected to my tv ?

Because I like to lay on my couch with my wireless keyboard and mouse and I find that the desktop is too small at normal setting. How would I go about setting the dpi to say 140 when my laptop is connected to my tv, then when I disconnect and just use my laptop screen the dpi would revert back to default settings ?

Is win 7 capable of increasing the setting automatically when it detects an external monitor then when I disconnect it would revert back to default setting ? Without having to change the setting all the time ?

Thanks