You're both correct-ish. The constraint on blocks is that they must "weigh" less than 4 million units. Most of time blocks that weigh that much are going to be

You're both correct-ish. The constraint on blocks is that they must "weigh" less than 4 million units. Most of time blocks that weigh that much are going to be around 1.4MB (or what ever) but in an absolute worst case would be a size of 4MB by a specially crafted transaction