Yes, the bigger a tumour gets the more likely it is to interfere with a vital process. Some tumours can kill when they are small, and others can grow to huge sizes without doing a lot of damage, but as a rule of thumb, the bigger it gets, the worse the damage it does.
Yes. If a tumour is large, then it can obstruct normal biological processes. For instance, if you have a tumour in your heart, it can obstruct one of the important arteries. Or, to give another example, a tumour in the pancreas could obstruct one of the important ducts that results in the organ not functioning properly.
In terms of weight, I can give an interesting anecdote to illustrate the kind of impact a big tumour can have one someone’s life. The world’s heaviest tumour was 137kg – about the same as nearly two full grown men. The woman who had it could not leave her bed, and the reason the tumour got so big was because she had agoraphobia, so she was too scared to leave the house.
So, whilst how aggressively malignant a tumour is certainly a big factor in the damage it can do – but so is the physical size of the tumour.