Only small in relation to the drying time one would face with acrylics. Small is one way to do a faster painting, and therefore you may get done before you loose your acrylics. But small is in a lot of ways harder to do, particularly with portraits.
Mark says to paint the same size you have in the source photograph. And he also says do not paint from a screen, but like you, that is my source. My local library has some Makerspace stuff that includes large scale printers. I don't know if they have laminators.
One trick to use if you do not have a large enough source, is to plot out key points within some program, and print those off, and lay them over your canvas, and produce dots. That way, you quickly get to the points, and you can ignore the role that having the perfect full size photo would provide, which is largely to transfer dots for the likeness. Or you can print a black and white or colour full scale, printer paper source. Then you can lay in your key points as Mark suggest. Then use your smaller source for the colours. It is the exact same "cheat" as using the scaling devices and full size photo. Just works if you don't have a photo.
The one thing is that Mark's process really only works if you stick to it religiously, with a few diversions that are entirely within the bounds of his system. Probably better to try one of the other systems if you want to diverge too much. I say that as the ultimate, diverger, instinctively. I am always looking for a work around. But if you look at the "first" paintings some of his followers have done, they only get there because they were slaves to the rhythm. And the method is slow and tedious, so not really worth it unless one gets results. As Mark has said, he would rather be doing woodwork in Mauii.
I should say I am a rank beginner at painting, but I have a huge background in crafts and some teaching, so the technical side is something I can understand and advance. I am a long way from being a painter.
Did you look at Mark's 3 main free videos? I borrowed one of Mark's videos that are for pay, god it was boring. Actually I think the video by the Portrait Institute guy on doing the Sargent painting, on Youtube is more helpful, and a little helpful relative to Mark's method. They both idolize Sargent. I borrowed one of that guy's videos that is not on YT, and he draws some live girl. What really came home was how bad the colours were compared to Mark. Same thing with Michael Greene's video that someone pirated to YT. Mark's colour matching is pretty dramatic.
I went to Dollarama. They have a tempered glass plate that is pretty good for a pallet. One side is rough, which is good for your background paint. It is like 4 dollars. When I scrape it down, a few flecks of paint stay in declivities on the smooth side. Doesn't bother me, but if it bothers you, look elsewhere.
I watched all of Mark's videos. And it is cool to watch the early ones, because I don't think he is using a single product from Geneva at that point, he had not yet released them. And you can probably get everything he is using from within the GTA. The guy painted two presidents and sells head and shoulders portraits for 30 K, he didn't have any of the new cool stuff. And cool guy that he is, he leaves all that old stuff up on the site. He knows that the DIY types like me are not buying from him anyway. He knows who the real spenders are. Very cool. I should note some of this knew approaches save time an money.
Sorry to go on...