Boats come in many different sizes andshapes and work in different areas from lakes and rivers to surf and Oceans...
Your boat is moved by winds and waves, but when you lose power, or steering and / or the wind grabs you ; nothing much is more important than your anchor.
There are many different types and designs of anchors but your boat needs at least two, or preferably three different types...
Your map will show rocks or coral (avoid anchoring over coral), mud, seaweed or gravel. When you drop your anchor you do so for one reason; to stop your boat changing position.
Given you cannot be sure of the water depth or bottom type of structural holding power that your anchor will encounter, I recommend three types of anchors.
Anchors tend to be heavy, but like computer memory it is false economy to go cheap; anchors are your best insurance policy when actually on the water, so do not scrimp on cost...
The first is a river anchor (closest to the boat's nylon anchor line), this is a round bell shape with chunks cut out of it; this anchor has great holding power in mud etc. Next is a heavy 'navy' type that drags and digs due to weight and at the end is a plow type that cuts into gravel sand etc. The middle heavy navy anchor keeps this plow being pulled horizontally and thus dug in with a firm bottom set. So each of these anchors work to its greatest advantage according to local conditions and help minimise each others weaknesses so every anchor works more effectively.
So what size anchor, simply find the largest anchor of each of those three types for your boats surface area; remember anchors need a different rule for effectiveness. So forget boat length focus on how much hull the wind and waves can push upon, and buy the biggest river, plow and navy anchor suggested.
String these three anchors along a chain, with six foot of chain between each type. The middle is the heavy type of bulk holding power of a navy type anchor; the end is the plow type that channels into to bottom and assists the heavy middle navy anchor.
The anchor closest to the boat is the round river anchor that is great in muddy conditions via suction holding power. This anchor also serves many other useful purposes since it tends to keep the anchor line pulling horizontally along the bottom so this keeps all the anchors digging in and setting into the bottom for holding power.
So no matter what bottom type you encounter, (understand anchors can get caught on cables, rocks and especially in coral) so no matter how bad the storm your boat stays put as well as is likely to be expected. This relates to the length of scope in your anchor line, close to shore or around other vessels you may need a short scope on your anchor lines.
Imagine you could throw your anchor stright down and have it hold? well any movement will just bounce the anchor along the bottom, so scope means the anchor line that lies at a 30 degree or 60 degree angle, this means the boat changes direction while you sleep as the wind or tide changes, now your boat swings around to hit an obstruction; you get the idea...
But the river anchor keeps the anchor in touch with the bottom so all the anchors work better with a horizontal pull., thus you can use less anchor line scope angle. Remember friction can cut nylon lines so use chain for its resistance along the bottom. Then nylon from chain up to the boat, (make sure all connections cannot wiggle loose due to wave action).
This nylon line is slightly elastic that reduces the pull on your boat and upon the anchor set in the bottom, plus it saves weight and is more convinient than chain. Plus as heavy wave action tend to pull the boat the nylon tends to stretch tight with rougher pulling on the boat adversely affecting both passengers as well as anchor set. However the river anchor tends to 'bounce' up and down to cushion some of this wave action roughness. Just make sure your length of anchor line scope keeps this river anchor from being bounced into the hull of your boat...
Plus the river anchor is round and ultra convinient as a simple lunch hook to stop somewhere mild for some fishing or to eat lunch etc. Sometimes that extra anchor allows two anchors apart at 180 degrees apart to keep the bow of the boat from moving in a circle around a single anchor line.
This is a simple introduction to the complexities of just one part of boating safety. One last tip to recover your anchors use a tip line attached to the back of the end anchor to 'unset' its holding power so you can haul up you anchors for more cruising ;)
Oh yeah; remember your big boat + big storm + three anchors = a ripped and shredded anchor line :)