I've tried the following sql on Ingres but it comes up with an error...

UPDATE size SET as=
(SELECT feature_detail
FROM details
WHERE mod_uuid = '11E874E9-65B7-DB96-1CF52F59557C4631' AND feature_uuid = '62006489-65B7-DB96-1334DDB7ACD83765')
WHERE mod_uuid='11E874E9-65B7-DB96-1CF52F59557C4631'

What should I change to make this work on Ingres?

Is size the table you are updating, or the field you want to update?

SQL format will be

update <table>
set <field> = ( select <field>
from <table>
where ... )

Note the result of the inner select must be a single value.