Average size of introns in eukaryotes ranges from:
Intron length varies enormously across eukaryotic genomes, reflecting balance between functional elements and splicing efficiency. Small nuclear introns in lower eukaryotes average 50-100 nucleotides, minimally containing 5' splice site, branchpoint and 3' AG. In mammals, average intron exceeds 3 kb, some spanning hundreds of kilobases such as dystrophin introns, harboring regulatory RNAs, enhancers and transposable elements. Lower size limit is dictated by spliceosome steric requirements of about 50 bases. Thus range extends from around 50 bp to several kilobases or larger, with median near 1
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Intron Size and Genome Architecture