[RPG] How to a warlock use enlarge/reduce without multiclassing

No, Warlock Spellslots do not count toward the Multiclass Spellcaster table

Spell Slots. You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table. - PHB p. 164

Warlock isn't included in that list. Thus you would only have 2 1st level spells as a level 19 warlock, level 1 wizard PC. You could cast level 1 warlock spells using your wizard slots and cast known level 1 wizard spells using your pact magic. Granted you would be able to cast those level 1 spells at maximum level (as per usual with pact magic) but its not going to benefit you in comparison to Eldritch Master which allows you to double your spells per day effectively.

For your specific character, you are correct.

However you left off the invocations (which can be quite invaluable and often act as a spell without needing a spell slot). I'll break this down specific to your character:

Cantrips: 2 base +3 from Pact of the Tome = 5 cantrips (+1 from Infernal Legacy covered below)

Spells: 4 known + Hellish Rebuke - these spells must be selected from the Warlock spell list. The Great Old one patron allows you to select from an expanded spell list which includes the spells listed under Great Old One on PHB pg. 110. This means you can select from these spells when you learn a new spell, not that you know them in addition to the ones you do select.

Invocations: 2 invocations, these do not count against spells known. There is no requirement to know the spell in advance, otherwise this would be addressed under prerequisites in the invocation. In fact, the Invocations have a different name, and you can infer that the name of the Invocation is the Warlock version of said spell, with the spell name under said Invocation being used as reference to it's capability. This is demonstrated through Invocations such as Armor of Shadows which is effectively Mage Armor without the spell slot or material component requirement. Another example is Book of Ancient Secrets, which besides letting you pick any 2 rituals from any class (wow!), it allows you to cast a lot of your Warlock spells as rituals so you don't have to expend a spell slot (which are invaluable as a Warlock).

Infernal Legacy: All spells known from this are separate and distinct from your spell slots and invocations. These do not use spell slots to cast but function on a specific recharge as indicated by the description under Infernal Legacy. This is separate and distinct because it is not a Warlock feature, but a Race feature and does not require spellcasting or pact magic in order to utilize (it's in effect the same thing as a Dragonborn breath weapon).