When you perform the grow action for the first time it automatically uses the units px as you started off with a null value. You will need to parse the value of .fontSize before you can perform arithmetic on it. Try this...
parseInt(a.style.fontSize.replace('px', ''));
to get a numerical value you can perform arithmetic on.
In full...
function Grow()
{
var a=document.getElementById(1);
// Get a number we can perform arithmetic on
size = parseInt(a.style.fontSize.replace('px',''));
// Additional check needed because original dimensions not specified
if (!isNaN(size)) { // If we now have a number we can use
a.style.fontSize=size+50;
} else { // Otherwise, set to 50 (assuming we are starting from 0px)
a.style.fontSize=50;
}
}