You will want a big board.
For most beginners the recommendation is about 9ft/3ft over their head, and same for you. However, you will want a board with more volume than most beginners - ideally somewhere around 90 Litres. Surfing in cold water you will have a thick wetsuit as well. Thus something a bit wider/thicker/longer than the standard 9ft longboard. So 24 inches wide, 3 1/2 thick, more like 9ft6 long than 9ft. That kind of range.
The obvious choice is the Walden mega magic. Its a pretty expensive board though; however have a look and it gives you an indication of what you should be thinking about
https://www.waldensurfboards.com/collec ... mega-magicAnother option is to go for a 9'6 longboard like the Torq
https://www.torq-surfboards.com/id-96-longboard.htmlor NSP
https://www.nspsurfboards.com/product/e ... longboard/ These are cheaper boards, perfectly fine for a beginner (NSP has multiple options).
Now there are heaps of longboard manufacturers/shapers around, so dont feel you have to go for one of the ones linked - just showing you the options and what you should look for. Keep in mind you ideally will want an 'oversized' board - so most longboards will be a bit too small (most are around 70L volume, you will want closer to 90L or more). Go longer, go wider
A possible option is a big softboard. Softboards have plenty of volume and might be ok for a while. However! Cheaper softboards and even some of the better brands will be too flexible for you (they will 'bend'). So I strongly suggest a hard board rather than a softboard; nonetheless if you wanted to start with a softboard that is an option. Again, 9ft or longer if you can find longer.
If you are interested in a new board, you may find a custom board is just as cheap - hunt around for someone who makes longboards and ask for one wider and thicker along the lines of the Walden.
Hope that helps