Telescope sizes are quoted in inches or millimetres and the numbers hide how quickly capability grows: light grasp goes with the square of the diameter, so each step up is larger than it sounds and heavier than people expect.
What each step gathers
A 200mm mirror gathers about 1.8 times as much light as a 150mm one, and a 250mm about 1.6 times as much as the 200mm. In practice a step of two inches is clearly visible on faint objects and barely visible on the moon and planets, where the atmosphere sets the limit long before the aperture does.
What each step weighs
This is where the decision is actually made. A 150mm dobsonian is carried in one hand. A 200mm is two hands or two trips. A 250mm is a heavy base and a bulky tube, and a 300mm is a project. The aperture that gets used is the one that goes outside without a decision being made about it.
The small end, in millimetres
70mm, 80mm and 100mm instruments are usually refractors and are bought for portability. A 100mm refractor gathers about as much light as a 100mm reflector and costs several times as much, which is the price of a sealed tube and no collimation.
Where the sweet spot sits
For most people in most homes, 150mm to 200mm. Below that the instrument is limited on faint objects; above it, the limitation is human. That range is where the majority of amateur observing is done, and it has been for decades.
Questions people ask about telescope
What size telescope should I buy?
150mm to 200mm for most people, which balances what it shows against what you will carry. Smaller if storage or portability is the binding constraint.
Is an 8 inch telescope better than a 6 inch?
It gathers about 1.8 times the light and resolves a third more detail, and it weighs noticeably more. On faint objects the difference is clear.
What does the number on a telescope mean?
The aperture: the diameter of the main mirror or lens, in inches or millimetres. It is the specification that decides what the instrument can show.