Permanent cotton fabric sealer (ground,gesso,size) for permanent adherence to magic smooth epoxy.

I believe that an acrylic dispersion medium designed as a size would work perfectly well for this purpose and should remain compatible. You should follow directions for sizing fabric intended for an oil ground. This should provide sufficient hold out.
However, I do wonder if you are purposefully avoiding applying a nacrylic dispersion ground (often called acrylic gesso) or if that is an option. First sizing with an acrylic medium and then multiple coats of such a ground would both seal, fill in the interstices, but also provide rigidity which could be very beneficial as the epoxy material you intend on using is likely to become quite brittle. It is generally best practice to apply more flexible materials over more rigid ones, rather than the other way around.

I will respond within your post for convenience. My responses will be in red.
Thank you very much for the information. I did try an acrylic dispersion ground from liquitex, but for some reason it did not adhere that well to the canvas, the epoxy could be pulled off a bit too easily.  Could the reason be that it was applied directly to the fabric prior to sizing it with an acrylic medium? or not the right thickness and amount of layers applied? 
This does not make sense to me. The acrylic dispersion ground should have adhered perfectly well to the canvas without a size. The size would just prevent the ground from soaking into the canvas. What kind of canvas are you using? Is this one specifically for artists or could it have some sort of commercial coating that is impeding adhesion?
For sizing you mention that it would be a sufficient hold out , do you mean permanent ? 
I simply meant enough of a size layer to prevent the epoxy from actually soaking into the fibers of the canvas.
Regarding the sizing , is an acrylic polymer (acrylic dispersion medium designed as size) more durable (archival) than a PVA size ? If yes could you please recommend the most durable ones in the market ?
I am not specifically saying that acrylic dispersions are more “archival”. My understanding is that they are capable of creating a more rigid surface which may be of use to you are you are applying a brittle epoxy layer to a flexible support and any added rigidity would help to offset this to some degree.
You recommended following directions for sizing fabric intended for an oil ground, is there a particular reason for this ?
I only meant that if you applied it in a similar manner as one used for an oil ground, the layer would be sufficient to prevent the epoxy from soaking into the canvas fibers.
Do you think that Size → Ground→ Epoxy is a permanent and best solution instead of Ground→Epoxy or Size→Epoxy ? 
I think that either a canvas with a size and a ground or even a substantial acrylic dispersion ground without a size would be preferable to the size/epoxy, as they would create a more rigid fabric surface than would a canvas with only a size. My worry is that a flimsy fabric with only a size would result in the cracking of the brittle epoxy over time.
If you think the Sizing → Ground → Epoxy is the best solution , could you recommend the best grounds + sizing for this application ? 
We at MITRA generally do not recommend specific manufacturers unless they make unique products and any profession grade of acrylic dispersion medium and ground should suffice. However, I am aware that when used as a size Golden’s GAC 400 is specifically formulated to create a stiffer fabric, which may be helpful in your situation. Their recommendations can be found here:
https://www.goldenpaints.com/technicalinfo_gac100s

Thank you in advance for your help. 
Best.
Tao