The font sizes you are seeing are dependent on the size defined by the HTML of the e-mail. If you cannot find an option to enlarge the font size in Thunderbird, try using Windows Key + and - in Windows 7 to make use of the magnifier for hard to read e-mails.
The problem of difficult to read font size is the result of monitor resolution changes taking place. While many users are now using high resolution, hi-definition monitors, some are still using older, low resolution monitors. This has resulted in a disparity that requires many publishers to display their content with a smaller font, in order to accomodate words that would appear too large on the screen.
The majority of Internet users are still not using hi-definition, high resolution monitors, but this number is growing, resulting in disproportioned content. It is recommended that you make use of the program's own magnification abilities, and the larger and smaller font sizes under Display Properties. When that is not possible, it is recommended that you use the magnifier option in Windows.
Microsoft MVP | MCITP | MCSA |MCTS |MCP | Network+ | A+ Certified
Operating System Windows 8 Pro x64
Computer Type Gigabyte GA-X58A-UD7 v1
OS Service Pack SP1 6.1.7601 (Win7 RTM)
Internet Explorer Version 10.0.9200.16384
DirectX Version 11
CPU Type and Speed Intel Core i7 Extreme 975, 3500 MHz (26 x 135)
CPU Cooling CoolerMaster Hyper 212 Plus RR-B10-212P-G1
CPU Idle Temp 43c
CPU Load Temp 60c
Motherboard Chipset Gigabyte Intel X58 + ICH10R GA-X58A-UD7
System BIOS Revision F8
System Memory Type 24GB G Skill F3-12800CL9-4GBRL
System Memory Speed DDR3-1333 (667MHz)
System Memory Clocking 8-8-8-22
Video Card Type and Speed SAPPHIRE 100315L Radeon HD 6850 1GB 256-bit GDDR5
Video Card Cooling Standard
Video Card Temperature 54c
Power Supply Unit (PSU) Rosewill BRONZE Series RBR1000-M 1000W
Computer Monitor Samsung SyncMaster P2770 27"
Sound Card Realtek ALC889 @ Intel 82801JB ICH10
Speakers Logitech 2.1
Headset/Microphone Sony MDR-V600/Yeti Mic
Storage Controller Intel IICH10R 3400 Series SATA RAID
Hard Drive x6 CRUCIAL_CT128M225
Optical Drives Portable USB DVD-RW
Keyboard and Mouse MS Keyboard / Logitech G500 Mouse
Modem-Router Type Cisco-Linksys WRT610N
Network Adapter x2 Realtek PCIe GBE Family Controller
Printer Canon MX850
Network Speed 1GBit Internet / LAN
USB Controller Intel 82801JB ICH10
Gaming Console PS3, XBox
Anti-virus Software ESET Smart Security
Productivity Suite Microsoft Office 2010
System Install Date 8/18/2012
Computer Skill Level Certified Professional
Windows Experience Index 7.7
Favorite Game Skyrim
Favorite Application Skype