Inside the TivaWare usblib/device/usbdcdc.c there are the following defines:

//*****************************************************************************
//
// Maximum packet size for the bulk endpoints used for serial data
// transmission and reception and the associated FIFO sizes to set aside
// for each endpoint.
//
//*****************************************************************************
#define DATA_IN_EP_MAX_SIZE     USBFIFOSizeToBytes(USB_FIFO_SZ_64)
#define DATA_OUT_EP_MAX_SIZE    USBFIFOSizeToBytes(USB_FIFO_SZ_64)

#define DATA_IN_EP_MAX_SIZE_HS  USBFIFOSizeToBytes(USB_FIFO_SZ_512)
#define DATA_OUT_EP_MAX_SIZE_HS USBFIFOSizeToBytes(USB_FIFO_SZ_512)

#define CTL_IN_EP_MAX_SIZE      USBFIFOSizeToBytes(USB_FIFO_SZ_16)

And the USBDCDCCompositeInit() functions has the following:

    //
    // Get the Feature set for ULPI
    //
    USBDCDFeatureGet(0, USBLIB_FEATURE_USBULPI, &ui32ulpiFeature);

    g_ui16MaxPacketSize = USBFIFOSizeToBytes(USB_FIFO_SZ_64);

    //
    // The CDC serial configuration is different for composite devices and
    // stand alone devices.
    //
    if(psCompEntry == 0)
    {
        psInst->sDevInfo.ppsConfigDescriptors = g_ppCDCSerConfigDescriptors;

        if(USBLIB_FEATURE_ULPI_HS == ui32ulpiFeature)
        {
            psInst->sDevInfo.ppsConfigDescriptors = g_ppCDCSerConfigDescriptorsHS;
            g_ui16MaxPacketSize = USBFIFOSizeToBytes(USB_FIFO_SZ_512);
        }
    }
    else
    {
        psInst->sDevInfo.ppsConfigDescriptors = g_pCDCCompSerConfigDescriptors;
        if(USBLIB_FEATURE_ULPI_HS == ui32ulpiFeature)
        {
            psInst->sDevInfo.ppsConfigDescriptors = g_pCDCCompSerConfigDescriptorsHS;
            g_ui16MaxPacketSize = USBFIFOSizeToBytes(USB_FIFO_SZ_512);
        }
    }
    psInst->sDevInfo.ppui8StringDescriptors = 0;
    psInst->sDevInfo.ui32NumStringDescriptors = 0;

And the USBDCDCPacketWrite() function has the following check:

    //
    // Can we send the data provided?
    //
    if((ui32Length > g_ui16MaxPacketSize) ||
       (psInst->iCDCTxState != eCDCStateIdle))
    {
        //
        // Either the packet was too big or we are in the middle of sending
        // another packet.  Return 0 to indicate that we can't send this data.
        //
        return(0);
    }

If the TM4C129 USB controller is using the internal phy then it is a USB full-speed (12 Mbits/s) and the above code will select a maximum packet size for CDC of 64 bytes.

To get a maximum packet size of 512 bytes requires an external PHY through ULPI interface to get a USB high speed connection (480 Mbits/s).