Sydney's typography options are located in Customize > Typography. From this panel, you can select if you want to customize the typography for the Headings or Body.
In each of these two sections, you will find the following options:
Font family and weight
Select your preferred font family and then pick the font-weight you want. Keep in mind that the options in the font-weight dropdown change when you change the font family. Have a look at the Google Fonts website to see the available font families.
Line height and letter spacing
When you're happy with your font family selection, you can further tweak it by using the Line height and Letter spacing options. You can also just use the defaults, as most font families will look pretty good like this.
Text transform and decoration
Text transform: default (none), capitalize, lowercase, uppercase.
Text decoration: default (none), underline, line-through.
As seen in the screenshots above, Sydney lets you customize the font sizes for the body as well as for all headings. Keep in mind that you can set different font sizes for each type of device (desktop, table, mobile) by clicking on the device icons:
Please remember: These are not the only font size options available in Sydney. You can find font size options for various elements in their respective sections.