I often change the width of those fields by simply using css.
Go to the page the login module is on, look at the source code and find the ID of the module - for example I can see mine says id="dnn_ctr2440_Login_Login_DNN_txtUsername"
Therefore I can add this to my skin.css or portal.css:
#dnn_ctr2440_Login_pnlLogin .NormalTextBox {width:180px !important}
And the fields will be 180px wide.
Alternatively you can edit the textboxes in this file
DesktopModules/AuthenticationServices/DNN/Login.ascx
and make the fields any width you like - no re-compile required. Just make sure you re-do the change each time you upgrade.
Rob