David Butler, who wrote the book "Explain Pain" with Dr Lorimer Moseley, told me that there was every reason to trial mirror theraapy with a larger mirror as I am doing. David also said he thought that good therapists should also be using brain laterality techniques before and with mirroring.
If you have a physiotherapist who isn't familiar with mirror work they can access information and ask questions on a forum for therapists at the noigroup web site.
It costs me nothing. I do it anywhere there's a suitable mirror and it works for me.
Seek professional advice before trying something new.