A fixed size gui is generally a bad idea because most forms have a user interface element that can sensibly be resized.
First consider a form that only contains two buttons. (A silly form, yeah, but for the sake of discussion we'll assume that it's the right thing for the job) When sized initially in the right way (appropriately for localization, e.g.), there's no good reason to make the form resizable. If you enlarged or mazimized the form, you'd only make the area to click the button bigger, but you'd be clicking a smaller area than the button to resize it, so there's no reason. In this case, it doesn't make sense for the form to be resizable because there's no user-benefit for adding the extra control.
Now consider a form that contains a listview. There are clear usability benefits to making this form resizable in both dimensions. The listview may contain more items than can be shown in a smaller area, both horizontally and vertically, so it makes sense that this form be fully resizable to allow the user to display as much or as little of the data as they want.
Every form control implicitly has certain degrees of freedom either by constraint or by convention. Conventionally, buttons don't resize, so they have no degrees of freedom (even though they can resize). Listviews and listboxes have two degrees of freedom, horizontal to display more data per row and vertical to display more rows. Single line textboxes have one degree of freedom, horizontal. Because they're single line, they don't expand vertically, but they do expand horizontally.
These degrees of freedom help you determine the layout of your form. Form elements that have degrees of freedom should resize in the appropriate direction when the form is resized. I prefer to design my forms with only one control that has two degrees of freedom in an area where areas are typically separated via splitters. I prefer to avoid the explicit table layout panel unless there's a very specific need for it because it's easy to make a form overly complex when there's a table layout to work with. The anchors in WinForms provide an extremely powerful and flexible abstraction for control layout, however, so I strongly encourage their usage.