@aimkbe,
You question is only related to this thread in that we are talking about the same plugin. You should have just started a new thread for this.
The issue you are having is really just a CSS issue. There may be properties that are inherited from another stylesheet, or you may just need to adjust some of the widths on the form elements.
Here is some information that may help you with customizing the CSS forms:
https://butlerblog.com/2011/07/25/customize-the-wp-members-stylesheet/
You only need to specify the location of the custom stylesheet in the plugin settings. But, if you’re up to it, a more elegant way of loading a custom stylesheet is wp_enqueue_style:
https://butlerblog.com/2012/02/20/loading-custom-stylesheets-with-wp_enqueue_style/
There are three stylesheets included in the plugin download – the default, and two others. This should give you a variety to start from. Ideally, you would create your own stylesheet to fully integrate with your theme, but these give you a good starting framework:
https://butlerblog.com/2012/02/19/new-css-stylesheets-included-in-wp-members/
Lastly, when tracking down style issues, I highly recommend Firebug (getfirebug.com). It’s a free browser plugin and allows you to right-click and “inspect element” which will show you what properties are applied to a given element and where they are coming from (good for tracking down inherited properties from other stylesheets).