[ecfisa]

Hola bulc.

Según entiendo, no podes cambiar el tamaño del font de un InputBox, pero nada impide hacerte una unidad con una función similar que sí lo permita.

Código Delphi [-]
(* 
  El siguiente código de ejemplo está basado en el original de Delphi (unit Dialogs.pas)  
*)
unit uMiMensaje;

interface

uses Windows, Graphics;

type
  TMiMensaje = class
  private
    FLabelFont, FEditFont: TFont;
    function GetAveCharSize(Canvas: TCanvas): TPoint;
    function InputQuery(const ACaption, APrompt: string;var Value: string): Boolean;
  public
    property LabelFont: TFont read FLabelFont write FLabelFont;
    property EditFont: TFont read FEditFont write FEditFont;
    constructor Create;
    function InputBox(const ACaption, APrompt, ADefault: string): string;
    destructor Destroy; override;
  end;

implementation

uses  SysUtils, CommDlg, Forms, Controls, StdCtrls;

constructor TMiMensaje.Create;
begin
  inherited Create;
  FLabelFont:= TFont.Create;
  FEditFont:= TFont.Create;
end;

function TMiMensaje.GetAveCharSize(Canvas: TCanvas): TPoint;
var
  I: Integer;
  Buffer: array[0..51] of Char;
begin
  for I := 0 to 25 do Buffer[i] := Chr(I + Ord('A'));
  for I := 0 to 25 do Buffer[I + 26] := Chr(I + Ord('a'));
  GetTextExtentPoint(Canvas.Handle, Buffer, 52, TSize(Result));
  Result.X := Result.X div 52;
end;

function TMiMensaje.InputQuery(const ACaption, APrompt: string;var Value: string): Boolean;
const
  SMsgDlgOK = 'OK';
  SMsgDlgCancel = 'Cancel';
var
  Form: TForm;
  Prompt: TLabel;
  Edit: TEdit;
  DialogUnits: TPoint;
  ButtonTop, ButtonWidth, ButtonHeight: Integer;
begin
  Result := False;
  Form := TForm.Create(Application);
  with Form do
    try
      Canvas.Font := Font;
      DialogUnits := GetAveCharSize(Canvas);
      BorderStyle := bsDialog;
      Caption := ACaption;
      ClientWidth := MulDiv(180, DialogUnits.X, 4);
      Position := poScreenCenter;
      Prompt := TLabel.Create(Form);
      with Prompt do
      begin
        Parent := Form;
        Font.Assign(FLabelFont);
        Caption := APrompt;
        Left := MulDiv(8, DialogUnits.X, 4);
        Top := MulDiv(8, DialogUnits.Y, 8);
        Constraints.MaxWidth := MulDiv(164, DialogUnits.X, 4);
        WordWrap := True;
      end;
      Edit := TEdit.Create(Form);
      with Edit do
      begin
        Parent := Form;
        Font.Assign(FEditFont);
        Left := Prompt.Left;
        Top := Prompt.Top + Prompt.Height + 5;
        Width := MulDiv(164, DialogUnits.X, 4);
        MaxLength := 255;
        Text := Value;
        SelectAll;
      end;
      ButtonTop := Edit.Top + Edit.Height + 15;
      ButtonWidth := MulDiv(50, DialogUnits.X, 4);
      ButtonHeight := MulDiv(14, DialogUnits.Y, 8);
      with TButton.Create(Form) do
      begin
        Parent := Form;
        Caption := SMsgDlgOK;
        ModalResult := mrOk;
        Default := True;
        SetBounds(MulDiv(38, DialogUnits.X, 4), ButtonTop, ButtonWidth,
          ButtonHeight);
      end;
      with TButton.Create(Form) do
      begin
        Parent := Form;
        Caption := SMsgDlgCancel;
        ModalResult := mrCancel;
        Cancel := True;
        SetBounds(MulDiv(92, DialogUnits.X, 4), Edit.Top + Edit.Height + 15,
          ButtonWidth, ButtonHeight);
        Form.ClientHeight := Top + Height + 13;
      end;
      if ShowModal = mrOk then
      begin
        Value := Edit.Text;
        Result := True;
      end;
    finally
      Form.Free;
    end;
end;

function TMiMensaje.InputBox(const ACaption, APrompt, ADefault: string): string;
begin
  Result := ADefault;
  InputQuery(ACaption, APrompt, Result);
end;

destructor TMiMensaje.Destroy;
begin
  FLabelFont.Free;
  FEditFont.Free;
  inherited;
end;

end.

Ejemplo de uso:

Código Delphi [-]
...
implementation

uses uMiMensaje;

procedure TForm1.Button1Click(Sender: TObject);
var
  sDato: string;
begin
  with TMiMensaje.Create do
  try
    LabelFont.Size:= 14;
    LabelFont.Name:= 'Segoe Script';
    EditFont.Size:= 9;
    EditFont.Name:= 'Segoe Print';
    sDato:= InputBox('Introduce el número base...', 'Número:', sDato);
    ShowMessage(sDato);
  finally
    Free;
  end;
end;

Saludos