CSS width always overrides the size attribute. Your issue is likely due to the container around the inputs. Likely either input-prepend or input-append CSS classes.
The CB Login module is designed after Joomla protostar template, which is Bootstrap 2 when using the Bootstrap module layout. The default module layout does not use Bootstrap. Ensure you've the appropriate layout selected. If your Joomla template is Bootstrap 3 you may need to make a layout override or CSS adjustments to the CB Login module.
The prepend and append usage for Bootstrap 2 is not very good with responsive design and I recommend just disabling the username and password input icons in your CB Login module to disable that functionality, which should cause it to display fine.