What would you call a "small" or "medium" language in regards to the number of speakers? I suppose a "big" language would be Mandarin, English, Spanish, Arabic. Small would be Greenlandic or Faroese. Would there be any convention on what a medium-sized language is? For instance, would you refer to Norwegian as a "small" language?
If you compare it with all the languages spoken worldwide, Norwegian is a fairly large language:
A small language would be something with <= 1000 speakers maybe, i.e. in the graph something below 10^3 on the y-axis, for example Pwapwâ, counting no more than 40 speakers according to Ethnologue:
Compared to that, Norwegian, being a national language with 4,741,780 speakers according to Ethnologue, is huge.
Totally depends on context, though. Of course, compared to English, Mandarin or Spanish, something like Norwegian might seem small. On the other hand, when talking about severely endangered languages which are only spoken by a handful of people, i.e. less than 100 or so, a language with several 1000 speakers would be not be considered very small.
I think you might draw the line at about the middle on the vertical scale in the graphs above; the languages in the rather dense area (around 10^4 speakers) would be considered medium-size compared world-wide, the ones above with several 10,000 speakers or more rather large, and the languages further below with less than ~1000 speakers would be considered small.
This also shows when looking at the distribution in a table:
As you can see, only 5% of langauges have a million speakers, only 1.3% have at least 10 million, and a mere 0.1% of languages worldwide has more than 100 million speakers. This is statistically nothing.
The slightly-below-median, with 44.4% cumulative, is at 10,000 <= n < 100,000. The other slightly-more-than-half counts less then 10,000 speakers.
Of course, Greenlandic or Faroese seems tiny compred to something like English or Mandarin with several hundred Million speakers, but these 0.1% (!) can not be taken as a reference of measurement. The vast majority of the world's ~7000 languages counts significantly less speakers, as the above table shows, and relative values like "small" and "large" should be normalized by the total distribution of speakers across languages rather than exceptionally extreme values like Spanish or Standard Arabic.
As a rule of thumb, if even a non-linguist knows about a language's existence (like Norwegian, Greenlandic or Faroese), it's probably relatively large ;)