How I can delete all node/field revisions?
There's not yet a stable release for Revision deletion but a D7 port is on its way (not sure if its functional at all).  

In this thread someone suggested using VBO to isolate revisions and delete them.

If you feel comfortable with the API, make a loop through all your nodes, retrieves the revisions using node_revision_list() and delete them using node_revision_delete().

As a last resort and if you like messing with the DB (like I do) you can delete revisions' rows in field_revision_* and node_revision tables. Try this:  

WARNING: not tested at all !

DELETE FROM field_revision_body
WHERE NOT EXISTS (
  SELECT NULL
  FROM node
  WHERE vid = revision_id
);

DELETE FROM node_revision
WHERE NOT EXISTS (
  SELECT NULL
  FROM node
  WHERE node.vid = node_revision.vid
);


You can find the complete list of your revisions' tables in the information_schema database:

SELECT DISTINCT TABLE_NAME 
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME LIKE 'field_revision%'
AND TABLE_SCHEMA = 'your_database';