Most of the time, manga is indeed read from top to bottom. This is a common convention. However, there could be some exceptions where the layout or the presentation might be different for artistic or design reasons.
The difference is significant. Popular manga tend to be more mainstream, have more merchandising, and get adapted into other media more frequently. Indie manga offer unique perspectives but might have limited reach.
The standard manga page size is kind of flexible. Commonly, it's approximately 7 inches wide and 10 inches tall. But there can be variations based on factors like the target audience or the specific manga series.
The size of the weights lifted in manga can be all over the place. In action and fantasy manga, you might see characters lifting impossibly huge weights. But in more realistic ones, they're more in line with what's physically possible for humans.