Dobsonians are sold by aperture and almost nothing else, which makes choosing one unusually straightforward: pick the largest you will genuinely carry outside, then check that the base fits where you will store it.
Six and eight inches
A 150mm dobsonian is the lightest full sized instrument and the one most often carried in a single hand. A 200mm gathers nearly twice as much light and is the size most often called the best all round telescope: heavy enough to notice, light enough that most people still take it out.
Ten and twelve inches
A 250mm shows noticeably more on galaxies and resolves globular clusters properly, and it is where the tube and base usually become two separate carries. A 300mm is a serious object: excellent under dark skies, and the size at which many owners find they observe less often than they did with a smaller instrument.
Fourteen inches and up
At 350mm and beyond the instruments are usually collapsible or truss designs, because a solid tube that long is unmanageable. They are wonderful and they are a commitment: storage, transport, a ladder to reach the eyepiece at high elevations, and a cool down time measured in hours.
How to choose in one question
Where will it live, and how many steps and doorways are between there and the sky? Answer that honestly and the size chooses itself. Every experienced observer knows somebody with a large instrument that has not been used in two years, and nobody who regrets an instrument they use every clear night.
Questions people ask about dobsonian
What size dobsonian should I buy?
The largest you will carry outside without deciding to. For most people that is 150mm to 200mm; with a car and a ground floor, 250mm.
Is a 10 inch dobsonian too big?
Not optically, and often practically. The tube and base are usually two carries, which for some homes ends the matter.
How much more does an 8 inch show than a 6 inch?
About 1.8 times the light and a third more resolution, which is clearly visible on faint objects and barely so on the moon.