connect,connecting or connected

Its OK as it is; connecting is OK too,
but boards can be of different sizes.

The part of the sentence after the word "they" is a list. It is a list of properties of the things referred to as "they", which are presumably items of classroom display equipment. They sound like electronic chalkboards (or "whiteboards" as they are sometimes called).

They are the size of a chalkboard.
They connect to the teacher's computer.
They can be activated by a pen.

Because it would be tedious and clumsy to write out "they" once for each item, we can just write it once and follow it with a list.

Each item in the the list would make a grammatical sentence if used alone after "They". Thus the verb "connect" (3rd person plural) is correct (indeed, is required) here.

The verb "to connect" present tense:

Singular

1. I connect
2. You connect
3. He connects

Plural

1. We connect
2. You connect
3. They connect <--------------------------------

Note that OmSigDAVID is incorrect, because "They connecting to the teacher's computer" is definitely not correct English!

When we include a list in a sentence, the usual rule is that we separate each item from the next using a comma, except for the last two which are separated by the word "and".

I like cats, dogs, birds and horses.

Mother is beautiful, kind, generous and wise.

Here are item one, item two and item three.

Thus the comma is appropriate, correct and necessary.

The very nature of David's "knowledge" never ceases to amaze me..

Oh dear! Clumsy editing by me. Sorry.

U r just hostile to personal liberty and individualism.

U r distorting the original sentence. U left out a word.
The sentence was:
"They ARE the size of a chalkboard . . . "

Therefore, "connecting" is OK, to wit:
"They are the size of a chalkboard,
connecting to the teacher's computer and can be activated by a pen."

I can only wry sadly at your patent silliness..

Omsigdavid is merely doing something that we in England call "arguing the toss", and on very insecure ground too. He does not care to be corrected. He is severely mistaken, and should be ignored in this case.

That is not true.
U can ALSO argue principle or fact, if u feel like it.

British English idiom: arguing simply to be difficult or contrary.