I need to to know what my artboard's size be to created for a trifold brochure. For my own reference I folded an A4 paper manually. So my question is will a horizontal A4 sized artboard will do to have the same thing on print?
You are probably trying to create a DL-sized item from an A4 landscape document set up as 2 pages. In which case yes, create 2 A4 landscape pages (artboards) and take the image below as your guide. Non-printing guidelines can be added where the dotted lines are, so you know where the folds will be made in production. There are 2 options here depending on how you want the folds to open.
Make sure you follow the logic of the folds as you will probably need to send a 2-page pdf with the folds shuffled. If using the 'roll-fold' type make sure you place the 97mm flap on the right side for the second A4 page (the second page will be measured in reverse).
For a ton of other articles and tutorials on this just google 'a4 dl folded' or 'a4 trifold'.