Here is how to check if the string "abcde" is 4 characters or more:
{{str ≥ len | abcde | 4
| Equal or longer.
| Shorter.
}}
Which returns this:
Equal or longer.
Templates have a problem handling parameter data that contains equal signs "=". But that is easily solved by using numbered parameters. Here we check if the string "ab=cde" is 100 characters or more:
{{str ≥ len
| 1 = ab=cde
| 2 = 100
| 3 = Equal "=" or longer.
| 4 = Shorter, not "=".
}}
Which returns this:
Shorter, not "=".
It probably is a good habit to always use the numbered parameters, since you might not know beforehand exactly how the data will look that your template will feed to this meta-template.
{{str ≥ len
| 1 = A string
| 2 = A length
| 3 = Data to return/render when "longer than or equally long".
| 4 = Data to return/render when "shorter than".
}}
Empty or undefined strings are considered to be of 0 length.
The potential alternative templates names {{str >= len}} or (for the opposite) {{str < len}} do not work in MediaWiki.
Templates have problems to handle parameter data that contains pipes "|", unless the pipe is inside another template {{name|param1}} or inside a piped link [[Help:Template|help]]. Thus templates can not handle wikitables as input unless you escape them by using the {{!}} template. This makes it hard to use wikitables as parameters to templates. Instead the usual solution is to use "HTML wikimarkup" for the table code, which is more robust.
Template documentation