I believe that the woed of God should always be used in the context of the verses around the one you are reading.
Mustard seed.
In Matt 13 it seems to be abnormally large and the fowls are clearly stated to be satan and his fallen angels. I believe it represents the earthly human church. Not the spirital church that is following after Christ first.
It started small and if you consider all the churchs called christian, has grown to cover most of the world. Many false teachers and false doctrines rest in it's branchs.

Now if you want to consider other verses.
Matt 17:19-21
Here the mustard seed represents the need for growing faith. The disciples could not cast out the demon because of unbelief. They needed much prayer and fasting. These things would focus you on Christ and thereby make faith grow.

Luke 17:5-6
Here the mustard seed is talked about in growing faith. (increasing your faith) It shows what is possable with faith that has no doubts. Although my faith is increasing by study of the bible, prayer, gathering at my church,ect. I can not say it has no doubts. It is just a sprout. Not even a bush yet.

If you will consider these verses you will see the mustard seed is used as a example of good and bad. Alway consider the verses around what you are reading to get the context of what is being talked about. This is why I personally hate proof text. Read one verse only and then say what it means. Easy to miss interpt that way. Always read 2 or 3 verses before the verse you want and 2 or 3 after or better yet read the whole chapter to get the context.
May Christ richly bless you as you study (not just read over) his word.