Java (JDK), 129 bytes
Saved 10 bytes thanks to @ceilingcat!
s->{int m=0,z=s.length()+1,a[]=new int[z];for(var x:s.split("[^a-zA-Z]+"))a[x.length()]++;for(;z-->0;m=a[z]>a[m]?z:m);return m;}; user
- 457
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Saved 10 bytes thanks to @ceilingcat!
s->{int m=0,z=s.length()+1,a[]=new int[z];for(var x:s.split("[^a-zA-Z]+"))a[x.length()]++;for(;z-->0;m=a[z]>a[m]?z:m);return m;};