1) In the current vanilla game you choose a single tileset that is used to draw every thing, by default. The wiki has a
repository of tilesets, though a few are not listed there. So, you can use a tileset set that has larger tiles which will make every thing larger, but the game will not display fewer than 80 columns and if you try to use too large of a tileset, the game will hav to scale it down. The ideal tile size is the max window size divided by 80, so for a 2560x1080 display that would be a tileset with 32 pixel wide tiles.
Now, you can tell the game to draw text using the bundled TrueTypeFont, either with an init file setting or by pressing F12. However the size at which that text is rendered is determined by the tileset and zoom level, so is usually less readable. Also, some screens do not display as the TTF, like the thoughts screen in fort mode.
The game can zoom the display, using the mouse wheel by default, which will scale the tileset tiles. Of course, scaling can cause the displayed tiles to look blurry, pixelated, or distorted, and is also limited to no fewer than 80 columns.
For non-vanilla there is the DFHack plugin TWBT (Text Will Be Text) which allows you to set different tilesets for text and non-text. However, I don't know what sort of limits there are on that and if you can set the text tileset to be larger than the non-text one. The easiest way to get TWBT is by using one of the
packs such as
PE's.
2) We don't know a lot about how the premium version works, but looking at the screenshots, it is using 32x32 graphics tiles and for text is using the 8x12 tileset that the current game defaults to. So, I would guess that there will be an option to use a larger text tileset, though there might a limit to how large of one can be used, maybe being limited to 32 pixels. It is also unknown whether the TTF functionality will remain, and last we heard the zoom functionality would
not remain.