只是猜测，但怎么样？

public async Task<IActionResult> Search([FromQuery, MaxLength(4000)] Guid[] EntityIds)

MaxLength属性应该做你想做的...从.Net 6资源：

/// <summary>
    ///     Initializes a new instance of the <see cref="MaxLengthAttribute" /> class.
    /// </summary>
    /// <param name="length">
    ///     The maximum allowable length of collection/string data.
    ///     Value must be greater than zero.
    /// </param>
    [RequiresUnreferencedCode(CountPropertyHelper.RequiresUnreferencedCodeMessage)]
    public MaxLengthAttribute(int length)
        : base(() => DefaultErrorMessageString)
    {
        Length = length;
    }