what is filed size/lenght when using database?

Question

what is filed size/lenght when using database

Answer 1

There will be a system table on your database that has a list of table names.
There will also be a system table that will have for each table a list of columns, their data type, length and decimal places if necessary.

Answer 2

if you mean 'field' size/length, then it is the amount of data you can put into that field - so if you only want to allow people to input names that are no longer than 5 letters, then you set its type to 'text' and the length to 5.