typedef void    (*void_fct_void)    (void);
typedef void    (*void_fct_int16)   (int16);
typedef void    (*void_fct_int32)   (int32);
typedef void    (*void_fct_2int32)  (int32, int32);

...

Uint32 address;
Uint16 sizeIn;

address = HW_Usb_Read_4Bytes();
sizeIn = HW_Usb_Read_1Byte();

switch(sizeIn) {
    case 0:
        ((void_fct_void) address)();
        break;
    case 2:
        ((void_fct_int16) address)(HW_Usb_Read_2Bytes());
        break;
    case 4:
        ((void_fct_int32) address)(HW_Usb_Read_4Bytes());
        break;
    case 8:
        ((void_fct_2int32) address)(HW_Usb_Read_4Bytes(), HW_Usb_Read_4Bytes());
        break;
}

Uint32 address;
Uint16 sizeIn;

address = HW_Usb_Read_4Bytes();
sizeIn = HW_Usb_Read_1Byte();

putNbytesOnCallStack(sizeIn); // magic function
((void_fct_void) address)();

__asm {
    push sizeIn
    ; You can also call your fp using asm:
    call address
    ; But it's not very safe because your compiler may not
    ; have saved its registries before the call
}