Is this sentence lexically ambiguous?

It is the second one. "Intellectually" would not be used in that way for the first one ("cleverly sizing me up" would be better) and also adverbs either immediately precede or follow the verb they are modifying.

As others have said, it would be understood to mean the latter, because of the context - people often size up intellect. People aren't often described as *using* intellectual skills to perform sizing up, and - if that was the meaning - they haven't said **what** they were sizing up, which would be strange. If you wanted to make it more clear, you could say “He spent the evening sizing up my intellect.”

He spent the evening intellectually sizing me up \-  this guy presumably an HR was assessing my capacity in a clever and meticulous way He spent the evening sizing me up intellectually \- He was assessing my intellectual capability to perform a job

I don't think he is presumably HR, more likely a romantic partner or rival. You are being evaluated for your intellect all evening.  He is likely doing that with his intellect, but the adverb is not modifying his action so the sentence does not mean he is doing it in a clever or intelligent way. Similarly, if he were sizing you up physically he'd be mentally comparing his size to yours, not busting out a measuring tape to physically measure you.

Why did you invent an HR job? There is no context to imply that at all