Which table are field instance widget settings stored? Is it ok to update these everytime a node is saved?
You can use field_info_instance() to get the details, and field_update_instance() to save them:

$entity_type = 'node';
$field_name = 'field_name';
$bundle = 'content_type';

// Load
$info = field_info_instance($entity_type, $field_name, $bundle);

// Get
$min = $info['settings']['min'];
$max = $info['settings']['max'];

// Set
$info['settings']['min'] = 1;
$info['settings']['max'] = 10;

// Save
field_update_instance($info);


I can't see any problem with updating those values during a mass import, but I can't see the benefit of doing it for every single node save. It would probably be better to run it once at the beginning/end of the import process.

You can use the same code in hook_node_view() to retrieve the values for display on the node page if you need them.