Translation length and size
tl;dr: You can estimate translation length using machine translation, and you should communicate maximum number of words or characters to the translator in advance.
In most cases, translation length does not matter. For example, there is no reason why a contract can not be 15 pages in Finnish and 25 in English. But sometimes there are technical or practical limitations to the translation length. User interface elements, such as button captions, need to be short, and in some cases, the visual space for the translation is fixed. Instructing the translator to keep the translation within a specific number of characters is usually enough to guarantee that the UI won't "break". In general, you should aim to design a user interface that is responsive. In some other cases, there are technical limitations, such as Apple's App Store limitations on keyword length and the maximum size for a description.
Machine translation gives a fairly good ballpark estimate for how long and how many words a translated text will have. For example, when translating from Chinese to English, the content length increases a lot. A character in Chinese might represent a whole word in English; the text written in Chinese is much more compact. In a case such as this, you may want to provide a maximum length for the translation, instructing the translator to summarize the translation if the maximum length is exceeded. Please pay close attention to possible length limitations for the translation in advance; this way, the translation process will be quicker and easier, because the content length will fit your expectations starting from the first delivery.
Tools to estimate translation length: