Julia, 111 bytes

r\n='A':'Z'⊆uppercase(r) ? count(isletter,r)=>r : n+1=>""
~S=minimum(S[i:j]\n for i=1:(n=length(S)),j=1:n)[2]

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• returns "" for falsy cases
• very ineffective (both time and memory)
• does not optimize for window size/length - only the letter length is considered
• -1 byte by changing n+1 for 2n ...