I cant remember the exact way..
but SOMETIMES u can choose ur game window size using Option --> Graphics Option (i think - the tab where u can pick between window mode or not) --> then there should be an option to choose ur resolution...
If u've got the option there, choose a smaller resolution
The options u're given depends on ur monitor/graphics card i think, everyone's option is different... so if u dont hv a choice.. i dont htink there's much u can do..
but if u've got the choice.. that's how u do it..
Here