Well as I said it is all arbitrary as what constitutes small depends on the person and situation. If I tell someone off the street who knows nothing about mining that I have 50 GPUs running in my house, you're right they are going to think 1:) that's a lot of GPUs and 2:) that I am crazy.
Now if I tell a fellow miner the same thing, I think most of them will consider that still "small" as it would only consist of 8-10 dedicated rigs with an average of 5-7 GPUs each. Putting this into the context of a mining "farm", I think that is still a small amount as many here are likely to fall into the medium to large categories.
I also considered the investment requirements and likely income aspects, as 50 GPUs will net you around $50/day or about $1.00 each (after power) in income (depending on many factors of course) which while a nice side income I still consider this small for most people. I am basing this on a US viewpoint, and realize in some parts of the world this may be a lot, but in the US at least it is probably not enough to live solely on.
As far as the investment aspect, simplifying this as I know there are other hardware expenses, but saying a roughly $200/GPU investment on a 50 GPU farm would be around a $10,000 investment. While this is a good deal of money, as far as investing it is probably considered small as you would not expect to make enough off of a $10k investment to live off of or retire on.
While with a medium farm you are looking at a $10,000-$30,000 investment, med-large $30,000-$60,000, large $60,000-$120,000 and so on. Again these are simply arbitrary buckets to lump investments into, but I think they will work for our purposes.