You’ll need to use an advanced filter (which is client side so not ideal for performance). This assumes you have product’s variants as a list on the Product type.
Search for Products(with as many constraints as possible):filtered (Advanced: This Product’s Variants:each item’s length:filtered:(This length < slider max and This length > slider min))
Another approach would be to have the Product as a field on the variant. Then do a constrained search for the variant:each item’s Product and filter on the product. Might be more performant depending on your DB setup.