Web measurements are a well-established methodology for assessing the security and privacy landscape of the Internet. However, existing top lists of popular websites commonly used as measurement targets are unlabeled and lack semantic information about the nature of the sites they include. This limitation makes targeted measurements challenging, as researchers often need to rely on ad-hoc techniques to bias their datasets toward specific categories of interest. In this paper, we investigate the use of Large Language Models (LLMs) as a means to enable targeted web measurement studies through their semantic understanding capabilities. Building on prior literature, we identify key website classification tasks relevant to web measurements and construct datasets to systematically evaluate the performance of different LLMs on these tasks. Our results demonstrate that LLMs may achieve strong performance across multiple classification scenarios. We then conduct LLM-assisted web measurement studies inspired by prior work and rigorously assess the validity of the resulting research inferences. Our results demonstrate that LLMs can serve as a practical tool for analyzing security and privacy trends on the Web.