How to read inline-CSS from Gutenberg block?

Turns out Gutenberg has another way of storing attribute values: the block’s comment delimiter:

If no attribute source is specified, the attribute will be saved to (and read from) the block’s comment delimiter.

source

When I inspected the database entry, I indeed found the size attribute there:

<!-- wp:XXX {"size":48} -->
    <div>...</div>
<!-- /wp:XXX -->

The problem was that the number 48 was formatted as a number in this comment delimiter, but defined as a string in the attributes section. Once I’ve changed that and removed all the selector-lines, it worked perfectly.

attributes: {
    sign: {
        type: 'string',
        source: 'text',
        default: '+'
    },
    size: {
        type: 'number',
        default: 16,
    }
},