Hi Helga Afonso,
You can use the regular exp in the validation microflow and restrict the attribute max length to 200.
Maximum Length: 200
Regular Expression: '^[a-zA-Z0-9 ]*$'
length($MyEntity/Attribute) <= 200 and regex('\\A[a-zA-Z0-9.,!? ]*\\z', $MyEntity/Attribute) is not recognizing regex