They're confident of this fact, not because they've done any research, or made any observations, but merely because Rashi reports it. Unfortunately, they've missed the boat. Rashi, as I've argued at length elsewhere on this blog, wrote his commentary to smooth out perceived anomalies of the text, and he usually accomplished this by citing friendly midrashim. When more than one midrash on the same topic is available, he usually picks one, sometimes even changing the wording or the location of the midrash so that it does a better job of solving whatever problem he's hoping to address. Examples abound, and several are discussed on this blog.
In the case of the barley seed midrash Rashi quotes the midrash accurately but provides us with only one of two opinions
Turn to BT Snahedrin 67b. You'll find an argument between R. Eliezer and R. Papa. R. Eliezer is of the opinion that magicians can produce creatures from nothing, but only if the creature is larger than a barley seed. When he shares this theory with R. Papa, however, it is met with what appears to be anger and disgust:
R. Papa said: By God! [this is an exclamation] he cannot produce even something as large as a camel.R. Papa goes on to say that the magicians had the power of summoning, but not of creating. R. Papa's view of R. Eliezer's theory is not recorded by Rashi, and is therefore ignored by the thousands, if not tens of thousands, of O.J so-called scholars who pride themselves on their Torah knowledge, but don't seem able to remember what they have presumably seen on the pages of the Talmud when they discuss Chumash.
What explains this? If you're like me you've seen this happen a thousand times. Some speaker will open a discussion on introduce a question with the usual formula, ie the words, "We know", as in "We know that the Egyptian magicians couldn't produce anything smaller than a barley seed."
But we don't know. Not even Jewish tradition says we know.
What we know from Jewish tradition is that R. Papa and R. Eliezer argued about it and that R. Papa was astounded at R. Eliezer's theory. The exchange described in Sanhedrin shouldn't give anyone confidence about the world of magic, not when two amoraim** are in such stark disagreement.
What we know from Jewish tradition is that some later authorities, the Rishonim, believed that magic is impossible and that the Egyptians were simply conjurers using slight of hand.
What we know from history is that no one has ever performed a real magic trick under laboratory conditions.
And what we know from our own experiences and observations is that there is no such thing as real magic.
So how do these so called Torah scholars allow themselves to say "we know" when they speak about magic? How do they permit school teachers to train our children to believe that they know anything at all about how magic works? Why in 2010 are there so many Orthodox Jews walking around who, based on nothing more than a completely misunderstood Rashi, imagine that they have something accurate to say about magic?
Haven't they at least seen Sanhedrin?
*Correction April 30, 2010: The post originally stated that one of the "facts" my friends confidently cite is that "Egyptian magic had no power over anything smaller than a barley seed." This was imprecise. The sentence should have said "...Egyptian magicians had no power to produce anything smaller than a barley seed," which is how I phrased it in paragraph 6.
**Correction April 30, 2010: The post originally stated that R. Papa and R. Eliezer were tannaim. They were amoraim.
Search for more information about Rashi and his use of midrashim at 4torah.com.