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Answer to the first question. I assume you wanted to make the time of buff smaller, if yes: in database it's PS_GameDefs Skills table, KeepTime value (in seconds). Or what do you mean by saying "minify the buff size"?
Answer to the second question is rather complicated. You need to know WHERE to inject and WHAT to inject. For example:
In this repo developer injects into packet sender and retrieves packets. Different developers are using different tools and languages, but mainly it's c++. You'll have to research, there are some examples in this forum. Hope it helps.