There are two kind of fields for length input. There is a simple textfield, where you can type the entire length value, and there is a textfield and unit selector version.

In the case of latter you can type the "value", and you should follow the number formatting rules only.

In the case of prior you should type the unit to. The unit means the well-known abbreviation of the unit, f.e.: "m" for "meter". You can type at most one space between the value and the unit. You can type multiple parts in one field, like "1yd -9 ft", and as you can see, you're allowed to type negative values, too, but this is depends on the context, since fields can permit negative values.