There is no specific size or resolution at which you must design letters. Whatever you drag on the Fontself panel will automatically be scaled to roughly match the size of most other fonts.
Adjust the scale of your font
To quickly resize all glyphs, simply press on the buttons Scale - or +.
Rolling over these buttons will also display sample glyphs below (set in Adobe's Source Sans), so you can check how your font size will fare compared to others.
Note: mind to look at both uppercase or lowercase letters as you have to decide which ones should match the height of the sample font.
Matching glyphs to a specific font size
Do you need to create a letter or icon that will show up at a specific size, like a 10 points high glyph when the font size is set to 12 points? Here is how you should proceed:
In your AI document, scale your glyph shapes so they match the targeted glyph size (i.e. 10pt)
Create a new dummy rectangle that has 70% the height of the font size (in this case: 0.7 x 12pt = 8.4pt)
Now in Fontself Maker, start a new font and drop/batch this rectangle as your first glyph. It's going to be used by Fontself to get the right scale for your font.
Finally, import the rest of your glyphs (and feel free to delete the dummy rectangle)
That's it. Wonder why you have to use this magic number of 70%? Keep reading...
All the metrics displayed in Fontself Maker are in units per em (aka UPM), a typographic unit related to the currently specified font size (i.e. when you use a font at 12 points, one em will be equal to 12 points).
Fonts generated in Fontself are based on a 1000 UPM.
Theoretically, all glyphs' points should fit within a virtual grid of 1000 units (this would include your highest capitals as well as the lowest descending shape).
However, to ensure there is no distortion on your shapes when you convert them into fonts, the actual position of each glyph is stored at a higher resolution.
Since you can draw letters of any arbitrary size and as Fontself Maker doesn't know how big are going to be your glyphs, it needs to guess how it will scale the first glyphs to fit them within the virtual typographic bounding box of 1000 UPM.
Whenever you start a new font and drop the first shapes, a scaling factor will be calculated based on the type of characters you drop:
When the first shapes you create are lowercase letters, the extension will try to fit their height around 480.
When first dropping capitals or any other character, the extension will try to fit their height around 700 (i.e 70% of 1000 UPM).
As you add more glyphs, they will all be imported to have a relative size. This is why two fonts created from the same shapes but in a different order can lead to different sizes when they are used at the same text size.
The good news is that you can now fix this.
Match the spacing, kerning or scale between different font files
To speed up your workflow and ensure font styles share similar features, you can match the size of two fonts:
By import spacing & kerning values from another font file.
By entering a specific capHeight or xHeight value to scale all glyphs accordingly.
Mind that capital glyphs are required in your font when you change the capHeight (and alternatively, lowercase glyphs are required to scale when editing the xHeight value) - otherwise, the option to Update & Scale Font will not be displayed.