This reference tells you how much space you should allocate for an account.

This only applies to accounts that don't use zero-copy. zero-copy uses repr(C) with a pointer cast, so there the C layout applies.

In addition to the space for the account data, you have to add 8 to the space constraint for Anchor's internal discriminator (see the example).

Types	Space in bytes	Details/Example
bool	1	would only require 1 bit but still uses 1 byte
u8/i8	1
u16/i16	2
u32/i32	4
u64/i64	8
u128/i128	16
[T;amount]	space(T) * amount	e.g. space([u16;32]) = 2 * 32 = 64
Pubkey	32
Vec<T>	4 + (space(T) * amount)	Account size is fixed so account should be initialized with sufficient space from the beginning
String	4 + length of string in bytes	Account size is fixed so account should be initialized with sufficient space from the beginning
Option<T>	1 + (space(T))
Enum	1 + Largest Variant Size	e.g. Enum { A, B { val: u8 }, C { val: u16 } } -> 1 + space(u16) = 3
f32	4	serialization will fail for NaN
f64	8	serialization will fail for NaN

Example

#[account]
pub struct MyData {
    pub val: u16,
    pub state: GameState,
    pub players: Vec<Pubkey> // we want to support up to 10 players
}


impl MyData {
    pub const MAX_SIZE: usize = 2 + (1 + 32) + (4 + 10 * 32);
}


#[derive(AnchorSerialize, AnchorDeserialize, Clone, PartialEq, Eq)]
pub enum GameState {
    Active,
    Tie,
    Won { winner: Pubkey },
}


#[derive(Accounts)]
pub struct InitializeMyData<'info> {
    // Note that we have to add 8 to the space for the internal anchor
    #[account(init, payer = signer, space = 8 + MyData::MAX_SIZE)]
    pub acc: Account<'info, MyData>,
    pub signer: Signer<'info>,
    pub system_program: Program<'info, System>
}