Naomi,
Just make your class smart enough to resize its internals at Init() time. Drop the container onto the form, set the container size to fit your form surface and let the container figure out how to juggle it's contents around when it first constructs at runtime. It doesn't really matter at design time that your listboxes and buttons don't "look right".
>Forgot if this problem was discussed here or not. Is it possible to create a container based class that would resize its components properly in design-time (in run-time it's easy).