How to change account size

You can change a program owned account's size with the use of realloc. realloc can resize an account up to 10KB. When you use realloc to increase the size of an account, you must transfer lamports in order to keep that account rent-exempt.
realloc.rs
use {
  crate::{
      instruction::WhitelistInstruction,
      state::WhiteListData,
  },
  borsh::{BorshDeserialize, BorshSerialize},
  solana_program::{
      account_info::{next_account_info, AccountInfo},
      entrypoint::ProgramResult,
      msg,
      program::invoke_signed,
      program::invoke,
      program_error::ProgramError,
      pubkey::Pubkey,
      sysvar::Sysvar,
      sysvar::rent::Rent,
      system_instruction,
  },
  std::convert::TryInto,
};

pub fn process_instruction(
  _program_id: &Pubkey,
  accounts: &[AccountInfo],
  input: &[u8],
) -> ProgramResult {
  // Length = BOOL + VEC + Pubkey * n (n = number of keys)
  const INITIAL_ACCOUNT_LEN: usize = 1 + 4 + 0 ;
  msg!("input: {:?}", input);

  let instruction = WhitelistInstruction::try_from_slice(input)?;

  let accounts_iter = &mut accounts.iter();

  let funding_account = next_account_info(accounts_iter)?;
  let pda_account = next_account_info(accounts_iter)?;
  let system_program = next_account_info(accounts_iter)?;

  match instruction {
    WhitelistInstruction::Initialize => {
      msg!("Initialize");

      let (pda, pda_bump) = Pubkey::find_program_address(
          &[
            b"customaddress",
            &funding_account.key.to_bytes(),
          ],
          _program_id,
      );

      let signers_seeds: &[&[u8]; 3] = &[
          b"customaddress",
          &funding_account.key.to_bytes(),
          &[pda_bump],
      ];

      if pda.ne(&pda_account.key) {
          return Err(ProgramError::InvalidAccountData);
      }

      let lamports_required = Rent::get()?.minimum_balance(INITIAL_ACCOUNT_LEN);
      let create_pda_account_ix = system_instruction::create_account(
          &funding_account.key,
          &pda_account.key,
          lamports_required,
          INITIAL_ACCOUNT_LEN.try_into().unwrap(),
          &_program_id,
      );

      invoke_signed(
          &create_pda_account_ix,
          &[
              funding_account.clone(),
              pda_account.clone(),
              system_program.clone(),
          ],
          &[signers_seeds],
      )?;

      let mut pda_account_state = WhiteListData::try_from_slice(&pda_account.data.borrow())?;

      pda_account_state.is_initialized = true;
      pda_account_state.white_list = Vec::new();
      pda_account_state.serialize(&mut &mut pda_account.data.borrow_mut()[..])?;
      Ok(())
    }
    WhitelistInstruction::AddKey { key } => {
      msg!("AddKey");

      let mut pda_account_state = WhiteListData::try_from_slice(&pda_account.data.borrow())?;

      if !pda_account_state.is_initialized {
          return Err(ProgramError::InvalidAccountData);
      }

      let new_size = pda_account.data.borrow().len() + 32;

      let rent = Rent::get()?;
      let new_minimum_balance = rent.minimum_balance(new_size);

      let lamports_diff = new_minimum_balance.saturating_sub(pda_account.lamports());
      invoke(
          &system_instruction::transfer(funding_account.key, pda_account.key, lamports_diff),
          &[
              funding_account.clone(),
              pda_account.clone(),
              system_program.clone(),
          ],
      )?;

      pda_account.realloc(new_size, false)?;

      pda_account_state.white_list.push(key);
      pda_account_state.serialize(&mut &mut pda_account.data.borrow_mut()[..])?;

      Ok(())
    }
  }
}