An international network of/for intelligent organisms
In principle there ought to be no limit to size - just from the biochemical point of view but maintaining sufficient humidity, hygiene and food becomes hard at large sizes. Square meter is totally possible and not too hard - I've grown on that scale but I tend to stick to 22cm square dishes - you can fill one of those from a starting piece of sclerotium in about six days.
If you want huge use paper rather than agar ( easier to manipulate ), I use chromatography paper which is basically a thick sheet of A0 blotting paper. Start in the middle and feed oats in a ring around the slime so it's constantly migrating outward. Oats do not need to be sterile I use them straight from the bag, scooped out by hand and watered directly on the paper using a thumb over the end of a bottle - twice a day by day 4. You'll end up with a big yellow cowpat of slime mould.
If you fold one of these cowpats in 4 and direct the open end of the resulting cone at a fresh piece of wet paper, the slime will migrate off - a few oats on the new paper will encourage it to move. You can merge a load of cultures like this to get to a very large slime.
One huge warning - there's a lot of mobile biomass in one of the large dishes, miss a feed and they're fully capable of leaving a dish in search of food, they can make one hell of a mess.