Since I'm building a generic model (I don't know what RDBMS I'll employ, though I'm using Access for now), I'm looking for a rule-of-thumb figure. I'm trying to avoid having to change field names later, depending upon what RDBMS I settle on.
2 bytes per character only in some character encodings (unicode?)
maximum length of column for viability as a key is (in my opinion) 255 bytes, preferably substantially less
maximum length of column name varies by database system (access 64, sql server 128, as little as 6 characters for a couple of older ones) but for practical purposes shouldn't be any longer than ~40 characters