Help with an Acer Netbook that shrinks yahoo mail?

I bought an Acer Netbook for my 80 year old mother who follows directions well. But she is in another state and I cannot figure out how to help her with this. The Yahoo Mail site on her netbook is so small that she needs a magnifying glass to see it. The rest of the programs and icons are normal font. Following any help instructions only changes the font on outgoing messages. We cannot find a way to enlarge the Yahoo Mail page so it is readable. Nothing on their help pages addresses this. Can anyone give us step by step guidance as to how to adjust this? Acer's tech support was beyond useless. Thanks to anyone who uses this netbook and knows how to change stuff!