How to change blog title size.

First go to design>edit html>expand widget

Now find this code given below. Use ctrl+F to search

#header h1 {
font: bold 180% Droid Serif,Serif;
letter-spacing: -1px;
margin: 0;
padding: 30px 32px 1px;
text-shadow: 0 1px 0 #FFFFFF;
text-transform: none;

Now change (font :bold 180%) to any size you want.
Thats it!