Is the WHERE clause in my export.json file limited to 4k characters like a normal SOQL statement?
I have an org that has thousands of records, and I want to only grab a specific set of those.
I will have all the ids that I can copy into my WHERE clause of export.json.
Can I do that and is there any limitation regarding the maximal length of SOQL statement defined in export.json file ?
Yes it is possible. SOQL queries used by SFDMU is the same query like a "normal" SOQL statement, so they have the same set of limitations.
Put list of your ids in WHERE with IN operator.
It's important to set master = true for your sObject (or omit this parameter)
Visit the below article regarding allowed total length of SOQL query: