In case you need to create in your site a very large number of custom fields - say, tree, four or five hundreds - at some time you will get this MySQL error:
MySQL ERROR:
Error Number: 1118
Description: Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. You have to change some columns to TEXT or BLOBs
Query: ALTER TABLE exp_weblog_data ADD COLUMN field_ft_550 varchar(40) NOT NULL default 'none'
This is a MySQL limitation as opposed to a specific ExpressionEngine limitation.
What can you do in case you run into this error? In essence you have three options:
1) to change architecture of your site so that it would net need so many custom fields;
2) to change all varchar fields in the exp_weblog_data table into tinytext fields;
3) to use Victor Gutierrez’s Fielder module fields instead of usual custom fields.
Related: Knowledge Base article How many custom fields can my installation support?
This problem was discussed in this forum thread.