Q: How to restrict window resize?
A: Subclass the window procedure and process this messsages:

   case WM_QUERYTRACKINFO: {
      WinDefWindowProc( hwnd,msg,mp1,mp2 );
      PTRACKINFO pti = (PTRACKINFO)mp2;
      pti->ptlMaxTrackSize.x = bitmap.size.cx + border.cx;
      pti->ptlMaxTrackSize.y = bitmap.size.cy + border.cy;
      return (PVOID)TRUE;
   }
 
   case WM_MINMAXFRAME:
      if ( ((PSWP)mp1)->fl & SWP_MAXIMIZE ) {
         PSWP pswp  = (PSWP)mp1;
         SHORT ytop = pswp->y + pswp->cy;
         pswp->cx   = min( pswp->cx,bitmap.size.cx+border.cx );
         pswp->cy   = min( pswp->cy,bitmap.size.cy+border.cy );
         pswp->y    = ytop - pswp->cy;
         return WinDefWindowProc( hwnd,msg,mp1,mp2 );
      }
      
   default: 
      return WinDefWindowProc( hwnd,msg,mp1,mp2 );