On traditional frames with a horizontal top tube, the seat post itself could be quite short compared with the lengths you see on downsloping frames.
On all my old road bikes the seat post was quite short, usually 160 mm. For safety, you want at least 100 mm of the post to be inside the frame tube, so on my old bike, the highest the seat could be adjusted was 60 mm above the seat tube. For a typical mountain bike, the lengths are much longer, often 350 mm or 400 mm. On my Rocky Mountain bike, I have a relatively small frame and I use a long 400 mm seat post (since I am 6 foot tall).