By using the .HasMaxLength() method, the maximum character count can be configured for a property.
using System.Data.Entity;    
// ..
public class PersonContext : DbContext
{
    // ..
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // ..
        modelBuilder.Entity<Person>()
                    .Property(t => t.Name)
                    .HasMaxLength(100);
    }
}
The resulting column with the specified column length: