Does the academic community perceive letters and short articles differently from longer articles?
There's an interesting study of ecology papers by Fox et al., finding that at least in that field, longer papers tend to be cited more. The explanation they advance, however, is that longer papers contain "more and a greater diversity of data and ideas," not that scientists feel more warmly about long papers than short ones. Also, the effect size they found was small, with less than a 2% increase in citations per 10% increase in page number. This suggests that padding out a short paper with extra analyses is not an efficient way to raise its profile.

(Short papers, of course, can be very influential: the first paper to accurately describe the structure of DNA was almost exactly one page.)