second roller 26 and the third roller 27 the size box 23 has an overflow wall 37 which extends parallel in front of a rear wall 36 and is of a height corresponding to the maximum filling level of the bath the size box 23 has an indirect heater with a heating panel 38 namely a steam pressure panel which has been inserted into the bottom 24 of the size box 23 the heating panel 38 has an upper plate 39 and a lower plate 40 which are connected with each other at defined places channels for example in a serpentine pattern for the heating medium are formed by these connections between the plates 39 and 40 further spot shaped connections for stabilizing the heating panel 38 can be provided it is important that a contact surface 41 of the upper panel 39 of the heating panel 38 which is oriented toward the interior of the sizing agent bath 23 is level and closes off the bottom 24 by means of an inner surface 42 see fig3 a steam feed line 43 with a valve 45 which is provided with a control 44 leads to the heating panel 38 embodied as a steam pressure panel a discharge line with a condenser 47 for discharging the heating medium namely the steam is connected to the heating panel 38 instead of being designed as a steam pressure panel the heating panel 38 can be designed as a hot oil panel or as an electrically heatable panel in place of two plates 39 40 which constitute conduits for the heating medium it is for example possible to provide heating.