Each stroke and its length in Chinese characters are very important. They can produce a totally different meaning if they were written wrongly. There are examples below to show how vastly different the meanings can end up to be:
日 and 曰 are essentially made up of the exactly the same strokes but one of them is thinner and taller than the other. Hence, the importance of writing the characters to the correct size is crucial in such words. There are more examples below that are self-explanatory. Some words end up having opposite meanings because of the lengthening of a single stroke.
wei4 (have not)
未 stands for the future when used in the context of 未来 wei4 lai2 (have not - come). But when used in the context of 未做完 wei4 zuo4 wan2 (have not - do - finish), it can be taken as 'have not' and it would mean "haven't finish doing".
刀 can be used in Chinese proverbs like 拔刀相助 ba2 dao1 xiang1 zhu4 (pluck - knife - each other - help); it means to whip out one's knife to help those in need. In modern terms, it simply means to help each other when in need.
力 can be used in proverbs like 力不从心 li4 bu4 cong2 xin1 (strength - not - from - heart); it means that strength to help does not come from the heart alone. It is normally used in contexts when one would like to do something, but does not have the means to.
土 can be used in phrases like 水土不合 shui3 tu3 bu4 he2 (water - soil - not - compatible); it does NOT mean that water and soil doesn't go together. When taken in a phrase as such, it means that one is unable to adapt to the enviromental conditions of a foreign place.
士 can be used together with 兵 (bing1) to mean 'soldier' as well. 士兵 = soldier.