“I would like to express a big congratulations...” Why ‘a’ before the plural form?

At school I was taught that before the plural form we don't use the articles a and an. So why do people use a before big congratulations?

Examples:

• A Big Congratulations to Dr. Wei Cheng on His Latest Publication! link
• A big congratulations to the Devs!
  They have put in an incredible amount of work this week to get this online up and running for all of us. They have handled communication with the community perfectly keeping us up to date ... link
• A big congratulations!
  We would like to say a massive congratulations to ‘Stumble’ and ‘Tripp’ for successfully completing a five day residential to become qualified regional Makaton trainers! link

• A Big Congratulations!
  Further to our article last month, we wanted to congratulate Jane Mason and Tom Wicks on their effort in Nottingham Life Cycle 5. Jane and her team completed her 50 miles, while Tom finished his 75 miles in the aid of raising money for dementia research. link

• A big congratulations to Hozier from Artists Den and Chase Sapphire Preferred for being nominated for "Favorite Artist- Alternative Rock" by the American Music Awards. link

plus many more found on the Internet.

The term congratulation remains a noun whether it is used as an interjection or not. The term love in the exclamation, “Love you!” is still a verb, so I cannot help but disagree with @Spencer's analysis

However, it is interesting that congratulations is far more common than the singular congratulation. To say to someone Congratulation on your wedding! would be odd, but if you send someone a letter of congratulation that is acceptable.

From (my hard copy) Oxford Advanced Learners Dictionary

congratulation /kənˌɡrætʃuˈleɪʃn/ noun 1 [U] the action of congratulating sb or of being congratulated: a speech of congratulation for the winner. 2 congratulations [pl] (a) words of congratulation: offer sb one's congratulations on his sucess. (b) (also infml congrats) (used as an interjection): Congratulations on passing your driving test!

My dictionary also informs me that the indefinite article a is used with an abstract uncountable noun when it is preceded by an adjective, e.g. We're looking for someone with a good knowledge of German.

Consequently, a big (and also a great big) is often collocated with welcome, and thank you, e.g.

• A big welcome to our special guest,...
• a big thank you to the hospital staff*

and the fixed expressions:

• a great many
• a good many
• a few people

are perfectly idiomatic in English, although ‘a’ precedes a plural noun.

I think native speakers would say a big congratulation if the singular form was idiomatic when congratulating someone, but it's simply not. Although the following are commonly said and heard in speeches, they do sound a little formal.

• Our warmest congratulations ...
• My heartiest congratulations on your promotion

Native speakers are just used to using the plural noun form

see also: "Congratulation" vs. "congratulations"

Just another one of those crazy things to trip over in English.

I would like to offer a big "Congratulations!" to Bill and Sasha.

The word congratulations here isn't a noun; instead, it's an interjection, made into a quotation and then stuck in a sentence in place of a noun.

The speaker is saying, in a roundabout way, that he would like to say "Congratulations!". Once. Thus making the article necessary.

The speaker could have said

A big "Way to go!" to Bill and Sasha

or, less awkwardly,

I would like to offer my congratulations to Bill and Sasha.

instead.