Let's say that I have a Construct that with an AC of 14, 45 hit points, but Damage Reduction of 3. 

The amount of damage this creature can do ranges from 4d6 with potential to prone as a trample ability to 2d8+4 also with potential to knock prone.

Not sure if making this Construct a Large or Huge size should affect the CR since that increases the range of one of its attacks.

Would this be a CR 4 or a CR 5 creature by itself?

With the data you give its cr 2. You need to give me more data, like if it has vulnerabilities, resistances/immunities, what its CON score is and attack bonus.

While I disagree with EJOD's proposed CR, I agree that some additional info is needed to make a firm judgement here.

With what you've given us, it sounds roughly in the region of CR4: the damage it deals is on par with some other medium Constructs at that level, and although your suggested hitpoints are a little low, the flat -3 to damage will probably offset it. You should definitely consider Condition and Damage Immunities, and whether it has any additional features like Magic Resistance - those will round it off, and make it a more satisfying enemy for your players to tackle.

There’s a whole section in chapter 9 of the DMG that breaks down how to calculate CR: (https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#CreatingaMonster). The bare minimum necessary to calculate it’s CR are:

• # of HP
• AC
• Highest potential DPR
• Attack Bonus & Save DCs
  
  &
  
  
• Special Traits it might have
• Damage Resistances/Imunities

Quote from IamSposta >>

There’s a whole section in chapter 9 of the DMG that breaks down how to calculate CR: (https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#CreatingaMonster). The bare minimum necessary to calculate it’s CR are:

• # of HP
• AC
• Highest potential DPR
• Attack Bonus & Save DCs
  
  &
  
  
• Special Traits it might have
• Damage Resistances/Imunities

      I don't have a DMG, that's why I'm asking here.

      AC and hp are already listed.

      As a construct, it's immune to almost every condition except Prone and Stunned.  As a construct, immune to poison and psychic damage.

       Resists piercing and fire damage.

       No special saving throws.

       I already know it has higher AC but lower hp than an elephant (CR 4). The difference is that this monster has Damage Reduction and it's attacks can hit multiple targets per round even though it doesn't have multi-attack. So it's hard for me to do an apples-to-apples comparison, if you get my meaning.

         Attack bonus is +7.

       It's weakness is clearly vs. ranged attacks since it has no ranged attacks of its own. The resistance to piercing damage partially makes up for that, I think.

I need a Constitution score to work out health. 

STR 20, CON 14, DEX 10

Looks CR 5. it has 150(10d10+20) hp as well. Hope that helps!