FancyMenu allows you to add player entities to screens, so you can display the client player or other players, including custom entities that do not represent a real player at all with a custom skin, name, cape and so on.
To show a "mirror" of the client player, just right-click the Player Entity element and enable Copy Client Player. This will copy the client player's skin, cape and name.
If you want to show another existing player, just right-click the element and set the Player Name to an existing player name. This will automatically show the skin, cape and name of that existing player, as long as you do not have a custom skin or cape set and Copy Client Player is disabled.
If you do not want to show a real player at all and instead want to fully customize the entities skin, cape and name, you can right-click the element. There are options to set a custom skin and cape texture. If a custom skin and cape is active, you can also set any player name you want without it copying the player name's skin if the player exists.
The Player Entity element has full support for customizing its pose, so in other words you can freely move all of its limbs, body parts, etc.
To do that, right-click the element and click on Player Pose. This will open a screen with sliders to configure the X/Y/Z rotation of all body parts.
The player pose settings have a normal mode where you can customize the rotations with sliders and there's also an advanced mode that allows a text input for all rotations with full placeholder support, which makes it possible to even animate the entity with a Ticker element that sets rotation variables!
In Minecraft 1.21.1+ you can simply use the normal resize grabbers of the element to scale the entity.
For older versions (1.21.0 and older), Player Entity elements do not support direct resizing via the resize grabbers. Instead you need to right-click the element and click on Scale. This allows you to set a scale for the element. The default one should be 30, so setting it to 60 for example makes the player twice as big as normal, setting it to 15 shows it at half the size and so on.
For Minecraft 1.21.1+, there is an extra mod needed for making Player Entity elements work. The mod is called "Fancy Entity Renderer" and is available on CurseForge and Modrinth.
If there is no build available yet for the Minecraft version you're using, it will most likely get released at a later point.
Please keep in mind that FER is not developed by Keksuccino, so he has no control over when builds get released.