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It depends on what you need it for - if you're carrying your laptop around every day, it's worth investing in a good rucksack with a padded compartment so the thing stays secure even when it gets knocked around. Brands like Samsonite or well-known outdoor manufacturers have solid options, but honestly there are tons of things on Amazon with great value for money too. The main thing is just making sure the padding isn't too thin and the straps are comfortable if you're carrying it for longer periods - tight shoulder straps are a nightmare.
How intensively do you use the rucksack - more like occasionally in everyday life or do you pack it regularly stuffed full? That makes a big difference when choosing.
What's important is that you really pay attention to the padding on the laptop compartment and don't just trust the marketing promises. Check whether the compartment is padded all around - some rucksacks only have padding on the top and bottom, the sides are then thin as paper. That's a real risk if the rucksack tips over or bumps into something. You should also feel the quality of the zips before you buy; cheap ones give up quickly when the rucksack is full.
As for size, I'd honestly say don't go too generous with the dimensions. A rucksack that's too large tempts you to stuff it full, and then even the best laptop buffer eventually ends up under pressure. Make sure the compartment fits exactly to your device's size - that way the laptop stays fixed and doesn't slide around. You can best tell good workmanship when you have the rucksack in your hand, not just from a photo online.
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