The best workaround I found:
- Set font size up to maximum 1296 pt
- Select text object
- Go to Effect > Distort & Transform > Transform...
- Check Preview box
- Scale up text object as you need
Now you can scale up text object to any size and keep it fully editable while font size is still 1296 pt. That's a good news, but the problem is that the text object it-self is still the same size it only appears bigger and if you'll hover the mouse pointer over it you'll see the actual size. Also if you'll switch to Outline view (Ctrl+Y) text object will appear without Transform... effect, so editing in Outline view becomes a guess work. And if you'll try to convert type to outlines you'll get actual 1296 pt size type outlined not the one you see. And that's where you can do the last try to fight the limits.
- Go to Object > Expand Appearance
Now bounding box will become adequate to the text object size, text and type properties will still be fully editable, moreover now in Outline view you'll see the object as you see it in Preview. And even outlining type will now create outlines from what you see. Woah! Perfect workaround? Not really...
Very annoying drawback of this workaround is that each time you try to transform (move, rotate, reflect, scale, shear or align) the text object enlarged with Transform... effect, the effect immediately evaporates and text object become 1296 points size again.
How you can deal with it:
- Expand Appearance only after you've made all necessary adjustments.
- Use Transform... effect again to adjust already expanded object in case you only need to move, scale or rotate your text object. Don't forget to Expand Appearance once again after applying the effect.
- You can down-scale the text object by decreasing font size down from 1296 pt. Also you can initially set font size to 1000 pt, roughly scale up text object with Transform... effect, apply effect and then fine-tune scaling by adjusting font size.
And here's another obvious option: as Cai mentioned, you can simply down-scale your whole document by 2 (or 3, or any number) and then increase document`s dpi by the same number. Cai gives an example: if your desirable document size is 3m x 1m at 150 dpi you can down-scale it to 1.5m x 0.5m, but use 300 dpi instead. Don't forget to mention this willful size mismatch if you share your project files with others.