I have a meade ETX-125 telescope and I have a solar filter, my question is, can I place a small disk in the centre to create an artificial solar eclipse in the hope of seeing corona ejections, if this is possible how big does the disk need to be?
You can't do this unless your telescope is of a type which has an internal focal plane prior to the last set of lenses. Think of it this way: if you are looking at the sun thru your car's windshield, any obscuration on the windshield simply blocks some of the incoming wavefront, which is (roughly) the Fourier Transform of the object's spatial intensity pattern. To block the sun's disk, you need to block light at the object or image plane, not in phase space.
This can be, and is, done in some of the specialized solar observatories such as one of the 'scopes in Hawaii.
For home use, if you are able to open up your imager (the CCD or similar array inside the camera) and physically block the pixels when the solar disk is imaged, you might get enough contrast to see the corona in the unblocked pixels.