。
When you say 100 meters as a distance, it's exactly 100 meters.
Literal
Distance [at-で] 100-meters [when-you-say-といったら] exactly 100-meters is-[copula-である].
A tautological/clarifying statement. ~といったら ('when it comes to, if you say X') is a topic-setting particle — here establishing '100 meters as a distance' as the topic. かっきり ('exactly, precisely, on the dot') is a mimetic-derived adverb for exactness. である is the formal copula. The sentence seems to be clarifying that measurements are literal.