can we put an upper bound on the length of proteins using evolutionary time scales?

Not really, most big proteins consist of 'domains' which fold up pretty independantly of each other (smaller proteins can vary quite a bit though). Titin is a ~30,000 amino acid protein in human muscle with ~500 repeats of the same 3 basic modules all laid out in a line... over evolutionary time you can shuffle these functional units around and make all kinds of interesting combinations.

Actually, the lab I'm working in recently had a problem with this. We optimized a gene to be read extremely fast by a ribosome while still producing exactly the same protein sequence (manipulating synonymous codons). But it turned out that when you have the actual protien molecule being extruded from the ribosome as rapidly as we had induced it to be, the normal independant folding of successive domains was disrupted - one domain didn't fully fold before the next domain started being extruded, they interacted, and the protein folded all wrong and didn't work despite having exactly the same sequence as the wild protein.