My first Game (Snake) and some Problems

Hi guys, thats my first experience with unity. Im working with unity for 4 days so not that long time Ive created a little basic Snake Game, without automatic movement, where i want to add some features later ... but i have already some problems:

1. my box collider does not work well , sometimes the snake head goes through the food object without triggering or the trigger gets activated too late

2. i want my snake grow faster but im using the list function to remember the position of my snakehead, to put the tail on this position , but my problem is that im not working with a 1x1 pixel snake. Thats why the snake tail appears in the snake head to 75%. One snake part is 500x500 pixel xD (i know its way too big, the game is already 30 mb , but im just testing because i want to play the game on fullscreen later with a large field and i want to keep space for later graphic updates.

Maybe someone can tell me what will be a good size for one snakepart?

Is there any way to let the snaketails connect at the end of my snakehead? If not how can i change the movement of the snake to larger steps , one step big as a snake part?

1. My food is sometimes spawning on the border and not between the borders, also i wish me that the food dont spawn at a part of the snake

ive uploaded my project so you can test for yourself : https://dropfile.to/9g9ms

the code by now:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
 
 
 public class SnakeScript : MonoBehaviour
 {
         
     public GameObject tailPrefab;
     public GameObject food;
     public Transform border_l;
     public Transform border_r;
     public Transform border_b;
     public Transform border_t;
 
     public float speed = 10f;
 
     bool eat = false;
 
     bool left = false;
     bool right = false;
     bool up = false;
     bool down = false;
 
     List<Transform> tail = new List<Transform>();
      
           
     void Start()
     {               
         Spawn();        
     }
 
 
     void Update()
     {
         Move();
     }
   
     void Move()
     {
       
         if (Input.GetKey(KeyCode.RightArrow) && left == false)
         {
             transform.Translate(Vector2.right * speed*Time.deltaTime);
             right = true;
             up = false;
             down = false;
 
         }
         else if (Input.GetKey(KeyCode.DownArrow) && up == false)
         {
             transform.Translate(Vector2.down * speed*Time.deltaTime);
             down = true;
             right = false;
             left = false;
 
         }
         else if (Input.GetKey(KeyCode.LeftArrow) && right == false)
         {
             transform.Translate(Vector2.left * speed*Time.deltaTime);
             left = true;
             up = false;
             down = false;
 
         }
         else if (Input.GetKey(KeyCode.UpArrow) && down == false)
         {
             transform.Translate(Vector2.up * speed* Time.deltaTime);
             up = true;
             left = false;
             right = false;            
 
         }
        
         Vector2 v = transform.position;
 
         if (Input.GetKey(KeyCode.UpArrow) && down == false || Input.GetKey(KeyCode.RightArrow) && left == false || Input.GetKey(KeyCode.DownArrow) && up == false || Input.GetKey(KeyCode.LeftArrow) && right == false)
         {
             if (eat)
             {                               
                 GameObject g = (GameObject)Instantiate(tailPrefab,v,Quaternion.identity);
                                                                                                                             
                 tail.Insert(0, g.transform);
                 
                 eat = false;
             }
            
             else if (tail.Count > 0)
             {                
                 tail.Last().position = v;
                 
                 tail.Insert(0, tail.Last());
                 tail.RemoveAt(tail.Count - 1);
 
             }
         }
     }
 
     public void Spawn()
     {
         int x = (int)Random.Range(border_l.position.x, border_r.position.x);
         int y = (int)Random.Range(border_t.position.y, border_b.position.y);
 
         Instantiate(food, new Vector2(x, y), Quaternion.identity);
     }
     
 
     void OnTriggerEnter2D(Collider2D coll)
     {
 
         if (coll.name.StartsWith("food_prefab"))
         {
             
             eat = true;
             Destroy(coll.gameObject);
             Spawn();            
             speed=speed+5f;
 
         }
 
     }
 
 }
 

sry for my bad english and my bad unity skills ^^ hope someone can help me !