Does this help you get closer?
| makeresults count=5
| streamstats count
| eval uri = CASE ( count==1,"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",count==2,"bbbbbbbbbb",count==3,"ccccccccccccccccccccccccccccccccccc",count==4,"dddddd",count==5,"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )
| fields uri
| regex uri="[a-z-A-Z0-9+/]{4}|[a-z-A-Z0-9+/]{3}=|[a-z-A-Z0-9+/]{2}=="
| eval uri_len = len(uri)
| where uri_len > 24
| stats count by uri
I'm using some self contained SPL to generate the data so this will need to be tweaked for your example
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