The field size in the database table gives you the basic ones...
describe mdl_user;
+-------------------+--------------+------+-----+-----------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------------+--------------+------+-----+-----------+----------------+
| id | bigint(10) | NO | PRI | NULL | auto_increment |
| auth | varchar(20) | NO | MUL | manual | |
| confirmed | tinyint(1) | NO | MUL | 0 | |
| policyagreed | tinyint(1) | NO | | 0 | |
| deleted | tinyint(1) | NO | MUL | 0 | |
| suspended | tinyint(1) | NO | | 0 | |
| mnethostid | bigint(10) | NO | MUL | 0 | |
| username | varchar(100) | NO | | | |
| password | varchar(255) | NO | | | |
| idnumber | varchar(255) | NO | MUL | | |
| firstname | varchar(100) | NO | MUL | | |
| lastname | varchar(100) | NO | MUL | | |
| email | varchar(100) | NO | MUL | | |
| emailstop | tinyint(1) | NO | | 0 | |
| icq | varchar(15) | NO | | | |
| skype | varchar(50) | NO | | | |
| yahoo | varchar(50) | NO | | | |
| aim | varchar(50) | NO | | | |
| msn | varchar(50) | NO | | | |
| phone1 | varchar(20) | NO | | | |
| phone2 | varchar(20) | NO | | | |
| institution | varchar(255) | NO | | | |
| department | varchar(255) | NO | | | |
| address | varchar(255) | NO | | | |
| city | varchar(120) | NO | MUL | | |
| country | varchar(2) | NO | MUL | | |
| lang | varchar(30) | NO | | en | |
| calendartype | varchar(30) | NO | | gregorian | |
| theme | varchar(50) | NO | | | |
| timezone | varchar(100) | NO | | 99 | |
| firstaccess | bigint(10) | NO | | 0 | |
| lastaccess | bigint(10) | NO | MUL | 0 | |
| lastlogin | bigint(10) | NO | | 0 | |
| currentlogin | bigint(10) | NO | | 0 | |
| lastip | varchar(45) | NO | | | |
| secret | varchar(15) | NO | | | |
| picture | bigint(10) | NO | | 0 | |
| url | varchar(255) | NO | | | |
| description | longtext | YES | | NULL | |
| descriptionformat | tinyint(2) | NO | | 1 | |
| mailformat | tinyint(1) | NO | | 1 | |
| maildigest | tinyint(1) | NO | | 0 | |
| maildisplay | tinyint(2) | NO | | 2 | |
| autosubscribe | tinyint(1) | NO | | 1 | |
| trackforums | tinyint(1) | NO | | 0 | |
| timecreated | bigint(10) | NO | | 0 | |
| timemodified | bigint(10) | NO | | 0 | |
| trustbitmask | bigint(10) | NO | | 0 | |
| imagealt | varchar(255) | YES | | NULL | |
| lastnamephonetic | varchar(255) | YES | MUL | NULL | |
| firstnamephonetic | varchar(255) | YES | MUL | NULL | |
| middlename | varchar(255) | YES | MUL | NULL | |
| alternatename | varchar(255) | YES | MUL | NULL | |
+-------------------+--------------+------+-----+-----------+----------------+
