Both usermeta.meta_value and postmeta.meta_value are stored as LONGTEXT. Here’s how the MySQL docs describe the size of a LONGTEXT data type:
“A TEXT column with a maximum length of 4,294,967,295 or 4GB (232 – 1)
characters. The effective maximum length is less if the value contains
multi-byte characters. The effective maximum length of LONGTEXT
columns also depends on the configured maximum packet size in the
client/server protocol and available memory.”