Well, if you don't mind making a pact with some powerful entity, you could get 2 levels of warlock. This will give you access to a few spells, some of which perhaps being interesting (both stats and flavor-wise) for your character. But most of all, this will give you access to 2 invocations.
One of these invocations should be Devil's Sight, which grants you the ability to see through any form of darkness, magical or otherwise, for up to 120 feet. This is more than enough to cover the 15 feet of the darkness spell. Cast it on your clothes, and bring terror to your enemies.
While this is a good strategy (generally favored by blade pact warlocks), it has some limitations. While under the effect of the darkness spell, your allies cannot see you. As such, any spell requiring line of sight that they would like to cast on you simply cannot work. Healing you might be problematic, as it requires your allies to be able to touch you. They might know that you are smack in the middle of that sphere of pure black darkness but, once they get inside, it's suddenly not so easy to find you anymore.
These limitations also affect your enemies, however. Any attack from the outside of the sphere should have disadvantage as your enemies, even if they can approximate your position, can't exactly see you to aim properly. Enemies inside the darkness should also suffer from disadvantage when attacking you, unless they possess some form of blindsight or a similar ability. When you attack them, you should have advantage on the roll since you are functionally invisible for them.
The biggest cost of this strategy is the fact that it requires multi-classing. While the lvl 19 and 20 abilities for monk would not really be missed (realistically, few campaigns will reach these levels), it will still set you back 2 levels in obtaining you other core monk powers. Note that doing this would get you 2 invocations, however, and that some of them can be quite interesting for a ninja-esque character:
Armor of Shadows (cast mage armor at will, which might be better than
using your wisdom for AC if its under 16),
Eldritch Sight (cast detect magic at will),
- Eyes of the Runekeeper (allows you to read anything, even languages
you don't know... useful for spying!),
- Gaze of Two Minds (share the eyes of a willing target, more spying!),
- Mask of Many Faces (disguise self at will!), etc...
TL;DR
Sacrifice 2 monk levels and multiclass as a warlock. Gain some minor but potentially useful spellcasting, as well as the ability to see through magical darkness and one more ability of your choice!
Unlike some previous editions in which the fireball will adjust based on the space available and spread down corridors if contained, 5th ed simply states that the fireball will take up the amount of space listed as 20' radius sphere and spread around corners to fill that space, never expanding to exceed that distance from the point of origin. This specifically breaks the line of sight rule on page 204.
So if cast at the entrance of a 10x10 room with a 5' corridor leading into it, it shall fill that room and spread 20' down the corridor.
Fireball PHB PG 242
The fire spreads around corners.
Best Answer
No, you can't
Unfortunately, spells only do what they say they do. They have a fixed area of effect, unless they say otherwise - if they do, usually they include "up to x feet" instead of "x feet".
For example, wall of fire spell description says
so you can make the wall shorter or smaller (in case ringed wall).
The darkness spell description says:
The description of the silence spell says:
Both don't say "up to", so you can't reduce the area of effect of the spells.
Silence area of effect won't move once cast, so your scenario casting silence around your shoes to move sneakily won't work well, because the silenced area won't stick to your shoes. Darkness however, can be cast onto an object, so you can cast darkness on an object and put it within a box, and only opens it when you need the darkness.
At most you can only position the area to exclude your allies, like targetting higher or farther etc. Remember that most spells require you to have line of sight to the center of the area of effect.
As an alternative to darkness, you can use dark star, a spell from Explorer's Guide to Wildemount. This spell acts both as darkness and silence combined with flexible up to 40 feet radius. It is a 8th-level spell and still static, so not really a substitute. (Thanks user68fd!)