To answer your question re: Semantic, here's a link to the table section
If there are no dimensions listed, you can use the Inspect panel for Chrome developer tools to get pixel dimensions to start with.
However, there's also another question here:
Do the current components sizes and styles suit the product you are working on?
Remember, as the designer, it's your responsibility to alter components where the framework falls short, and the user is not well served by defaults. Test your designs with users to make sure.
Another thought here is an opinion based on years of prototyping experience (I know, it's my 2 cents).
Use a different tool than photoshop when designing large sequences of screens
Photoshop is best used for image manipulation. Here's some of the problems with photoshop for UX design:
Photoshop has a single document interface.
Interactions take place across time: this means to convey an interaction there are multiple state changes on the same page, or flowing across multiple pages.
You need a tool to view multiple documents at once. Panning and zooming to get context of your interactions as you work. Photoshop can use Artboards, but other tools are more nimble.
Photoshop is not as nimble with programmatic thinking for design
You are using a framework to develop your actual interface. You should be thinking of ways to design components that are easy to update.
There are several tools which many UI and UX designers favor.
All the designers I work with use Sketch. (NOTE: I am not being paid, nor do I work for this company). There are tons of user generated plugins available for using and arranging symbols, style guides, etc. All the things you'll be responsible for.
UPDATE: As mentioned in the comments above by @plainclothes, Balsamiq (available here for quick mocks) is great when you are not responsible for a detailed visual design layer, and want to work through a prototype quickly.
My experience is that it solves the multi-document view, where you have unlimited canvas to layout and pan across as you build view sizes and/or interactions.
This last part of the answer is perhaps a personal rant, but it comes from the problems I've encountered that have slowed teams (and individual designers) down.