The size argument is derived automatically from the data buffer you pass. Such buffers are called auto-sized buffers in LWJGL. All bindings work this way and the benefit is less bugs, less crashes and less things for the developer to worry about.
Functions that do such signature transformations are also exposed with an 'n' prefix (the 'n' means "unsafe") and perfectly match the native signature (with pointer types replaced by Java long of course). So if you need raw access to the native function, it is possible in LWJGL.
You can read more information in the
Bindings FAQ of the LWJGL wiki.