There is a very common word in Latin that literally means "two and a half": sestertium, -i which. This comes from semis + tertius, the idea being (I suppose) that it is "half-way to three [from two]."

This usage is antiquated and almost entirely replaced by the current meaning of a "serterce," which according to L&S is:

a small silver coin, originally equal to two and a half asses

L&S also quotes sestertium as meaning "two and a half feet [deep]," as used by Columella in De Arboribus:

ipsum agrum sat erit bipalio vertere: quod vocant rustici sestertium. (1)

...but this seems like sloppy research to me, since the sentence that immediately follows suggests a number more like 1 2/3:

...Ea repastinatio altitudinis habet plus sesquipedem, minus tamen quam duos pedes.

If you wish to use a prefix for "two and a half" then, you could feasibly go with sester-. It turns out that this phrase already has some traction, for instance as applied to a 250 year anniversary: "sestercentennial". See, for example, this New York Times article, this Wikipedia article, and ~28K Google results. These are all decidedly non-authoritative.

Alternatives

The above-cited Wikipedia article offers one other alternative that fits this case: semiquin-, which means "half of five."

A Word of Caution

The ultimate answer to your question is that there is no frequently used, easily understood prefix for "two and a half." This is probably because (!) it's not a terribly useful thing to have.

Classical attestations of any of the above suggestions are conspicuously absent, whereas we have from Varro a very clean example of what a Roman might have done when faced with the same question. Does he use "sesterpedis" to refer to a two-and-a-half-foot distance? Let's look:

...tympanum in latitudinem duo pedes et semipedem, in altitudinem palmum. (Varro, Res Rusticae 3.5.15.5)

So much easier to understand!