@Lisa The example you have posted is a poor example (not on your part) of a website wrongly using forms as a method of navigation. It is not what a HTML select menu is designed to do; and therefore a number of potential usability and accessibility problems emerge. Typically a select menu is only used inside a contact form or similar, to present a list of choices.
It would be wiser to have a proper menu (like an unordered list) that can be clicked and opens to reveal a list of links. Something like MyMenu could work: https://seydesign.com/stacks/mymenu/
Or even a simple toggle stack (e.g. https://stacks4stacks.com/toggle) with custom CSS to give the revealed panel a fixed height and to make it scroll:
max-height: 200px !important;
overflow: auto !important;
A list of 100 items or more could still be very long and tedious for a person to scroll through. I’d imagine it wouldn’t be terribly user friendly on touch devices. And the absence of scroll bars in some web browsers could bring the risk that people don’t realise your list of page links is scrollable. Have you considered a sitemap page, search box or live filtering system?