Hum... well resizing the width should be as trivial as modifying the width of the inputField.
Except if your inputField is under a layout. If that's the case, you'll find a "Manual Preferred Width" option on the InputField that you can play with ;)
Same thing for the height, you might want to untoggle "Fit Height to Content" and specify the "Manual Preferred Height" :)
If you want to change the size of the hint text, there's also a "Floating Hint" "Font Size" parameter that you can use on the InputField (in the inspector).
As with dealing with the size of the text itself, you can directly modify the InputText GameObject under the Input Field GameObject.
Does that answer everything? :)