ppt presentation for postdoc position interview I have to prepare the presentation about my previous research experience for 20 minutes. I don't know what I should include, how big and how  much specific it should be
The first suggestion: Ask the people interviewing you. There's nothing wrong with asking if they would like a deep-dive or a broader research context, or what kind of audience you'll be drawing from.

In a more general sense, if you Google academic job talks, you should get a fair amount of advice. In general, the goal is to present your previous research in a coherent narrative, establishing who you are as a researcher so they can put you in context. Tell a story, and supplement it with the results of your research. As it's only 20 minutes, my guess is they are intending it to be fairly high level, rather than a very detailed exploration of any one result.

As for "How Big"...it should be 20 minutes, and as big or small as you can cover in that time.