WidestFitDimensions[
  targetWidth_Integer,
  columnSpacings_Integer,
  leftRightBorderWidths:{_Integer,_Integer},
  data:{__List?MatrixQ}]:=
  With[
    {widths=Last/@Dimensions/@data,
     fullWidthOfRow=Total[ArrayPad[Riffle[#,columnSpacings],leftRightBorderWidths,1]]&},
    With[
      {fullWidthOfGrid=Max[fullWidthOfRow/@#]&},
      With[
        {isTooLarge=(targetWidth<fullWidthOfGrid[#])&},
        With[
          {bestFitGrid=
             NestWhile[Partition[widths,-1+Last@Dimensions@#,-1+Last@Dimensions@#,{1,1},0]&,
               {widths},isTooLarge[#]&,1,-1+Length@widths]},
          <|"dimensions"->Dimensions@bestFitGrid,"width"->fullWidthOfGrid@bestFitGrid|>]]]];