There is a very common word in Latin that literally means "two and a half": [***sestertium, -i***](http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dsestertius). This comes from [*semis*](http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dsemis) + *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](https://en.wikipedia.org/wiki/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](http://www.nytimes.com/1981/12/13/magazine/on-language-by-william-safire-woodshed-blues-kimble-mead.html?pagewanted=all), this [Wikipedia article](https://en.wikipedia.org/wiki/Anniversary), and [~28K Google results](https://www.google.com/search?q=sestercentennial). 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!