The full form should probably be 「さぁ…さっぱりわからない」, but is it OK to omit the latter part in informal conversations and will that cause any confusion or will it sound strange? I suppose it should be OK but I just want to make sure because there seem to be multiple explanations to さっぱり。
さっぱり mainly means:
- refreshed; neat; clean
- (as a negative polarity item) completely (not); entirely (not); (not) at all
Please read the link. Since さっぱり is an NPI like 全然 and ちっとも, the latter half of the sentence including ない can often be omitted.
In many cases, this signal is strong enough that you can leave out the actual part of the predicate containing the negation (as long as it can be inferred from context)
さっぱり can be さっぱり売れない, さっぱり聞こえない or anything, depending on the context.
すっかり is not an NPI, so saying only すっかり can often be confusing. Also note that さっぱりと (with と) is not an NPI.
Insofar as the other party understands the context (in response to a request for information or opinion), it is not uncommon to omit the latter part in casual conversation. I can only provide anecdotal evidence, as I couldn't find a source.
The same holds true for あー、すっきり（した）to convey a feeling of relief or being refreshed.