Hello.

I am not familiar with your custom web part and with its content, however on Portal engine page, you can manage it via WebPartZones in page layout and their dimension / positions. I would recommend you to inspect some existing page template (e.g. Home page with top image and three columns) and see, what styles and positioning is used. You can also experience that you can have 1 web part (e.g. standard Repeater), which have dynamic width, according to current WebPartZone (so it fits to width of given WebPartZone).

Of course, you can ensure fix dimension in Visual Studio, in Design mode.

I would recommend you, beside investigation of current page templates and styles on default Corporate site web site, to read following tutorial, where you can see more details: http://devnet.kentico.com/docs/tutorial/index.html

Best Regards,
Radek Macalik