As you can see by this image of the Pope, 「大」 is yet another one of those simple Kanji that is easy to memorize AND visualize.
The only tricky part is figuring out which reading to use in Kanji compounds. There is no rhythm nor reason for why 「大学」 is read as 「だい」 vs 「大使館」 as 「たい」 except perhaps those readings roll easier on the tongue. If you’re new to Kanji, you’ll see this is often the case for common characters with multiple readings.
As for 「大きな」, this is one of those funny adjectives that you can’t conjugate and only use as is. Stick with the i-adjective 「大きい」 for conjugations eg 「大きくない」 NOT 「大きじゃない」.
A: What are (you) saying? There’s no way I can say a Kanji in conversation, right? Or what? Are you concluding that what I’m saying materializes and is visible to the eye? It’s not like this is a Manga.
You might be asking why is mouth a square, not a circle? Honestly, I have no frickin’ clue. Oddly enough, circle is a shape that is not used in Kanji （○ is a symbol, not Kanji）. Even the Kanji for “circle” （丸） is not round in the slightest! Maybe something to do with how brush strokes work, I dunno.
Anyway, it is what it is, a square to symbolize a big, open mouth. Take EXTRA care to learn the stroke order because this is also a very important radical that will be used in quite a few other characters.
Also, this is visually identical to the Katakana: ロ but totally different OBVIOUSLY. (Rolls eyes)
Well, the easy ride is over but it was great while it lasted, wasn’t it? 「一二三」, let’s see, that’s 3 out of about 2,000+ characters so… progress?
There’s several explanations on how 「四」 came to be and most of them involve the fact that it comes from a combination of: 口＋八. However, the inside part more clearly resembles the 「儿」 leg radical (note: this radical is not a kanji by itself). If these Kanji are new to you and you prefer to learn in radical order, you may want to skip to the next suggestions.
One suggested memonic was an image of an open mouth to signify that your breath is impossible to count (one, two, three, many). This one must go back to the caveman days where four was considered a bigly number? In any case, this Kanji is still common and simple enough you could probably memorize it by brute repetition anyway. Hmph!
This one is pretty easy and given my (lack) of drawing ability, this is what my pathetic attempts to draw a person typically look like. My drawing ability is so bad, xkcd practically looks like Monet in comparison.
However, don’t be lulled into a false sense of security as the tricky part is remembering whether to read it as 「じん」 eg 「日本人、未亡人」 or 「にん」 eg 「職人」. Not to mention some of those tricky words with readings completely divorced from the actual characters: 「大人、一人、二人、仲人、若人、玄人」 etc. Don’t worry about them if you’re still relatively new to Kanji for now.