<p>How do i make the plus minus icons bigger in the tree view in C# .net with out making visual styles disable.</p>

I don't think you have any control about it. The only thing you can do is override the DrawNode event and paint the nodes yourself.
First you need to change the DrawMode to OwnerDrawAll. Take a look at the relevant msdn page, it contains surprisingly much detail:
http://msdn.microsoft.com/en-us/library/system.windows.forms.treeview.drawnode.aspx