I think confluence yet to have the feature to adjust the size of the profile picture.
As for now, all we can do is to vote for the suggestion that are posted in Atlassian Page and hope Atlassian developer will add on the feature Below are the link
As for now, there's a workaround that i can think of, is by using HTML macro.
If you're using this method, you would need to manually update the page if there's a changes in the profile picture.
Enter the code below in the HTML macro
<img src="<Your-Profile-Picture-Link>" alt="Profile Picture" style="width:256px;height:auto;">
Change the <Your-Profile-Picture-Link> into the profile picture you want to show (Step 3).
Modify the width pixel in style="width:256px;height:auto;" into the width/size that you want.
To Get the Profile Picture Link
Open a new Browser Tab
Hopefully it helps
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