Answer
Output Channels can be raised up to a hard ceiling of 100,000 rows (via Options > Server > Output Channels), but that's the maximum — it can't be increased further. The cap exists because Output Channel exports are processed on the client side: all of the formatting and reassembly specified in the output channel happens locally, so performance degrades sharply as row counts grow, and Output Channels were only ever designed for ad hoc, moderate-sized exports rather than bulk data extraction. For genuinely large volumes, a SQL Quick Report's built-in Export to Excel is the better tool — that export is generated server-side, which is substantially faster and doesn't hit the same client-side bottleneck.