Surfing Waves

I am new to surfing, and my wife decided she wanted to join me. We rented her a board and she decided she likes it, so she wants to be one. She is 5ft tall and we were wondering what size and width she should get. The board we rented her was 7'6" and 21" wide and 3 1/2" thick. She was not able to carry the board and had a hard time paddling because the board was to wide.
The distance from her armpit to her second knuckle was 17". I measured this to find out how wide it would have to be for her to carry it. I then took that measurement across her chest and it goes about 1/4 of the way into her bicep. This was about the same distance that my board comes across my chest. I just wasn't sure if this was too narrow for her to be able to keep balance on. Any suggestions would be appreciated. Thanks, 2 newbs.

my boards about 22" across and im ok with it, 17" is really quite small-perhaps go to a shaper/expert and ask them whats best/if they have anything in between. good luck!

how tall are you. My wife is only 5ft. I am 6ft and my board is 23 1/2" wide.

To the person who asked about optimum board width: I realise it has been a month since your post and you have probably figured it out, but in case you are still seeking advice, I would like to suggest that you buy a wide (21-22") board for stability when beginning and carry it overhead, or rest it on a hip or shoulder if it is too wide for under-arm carrying. You want the stability, don't sacrifice it for under arm carrying comfort. You'll not regret the additional width when learning.
Good luck!