As far as I know, t5 come in standard sizes to fit standard fixtures, like 24", 36", 48", etc. You can also tell he size of the bulb by it's wattage, like 54w is a 48" t5ho bulb.
Look at your bulbs, there should be a brand and wattage printed on one end.
I find that drsforrestsmith com have a good selection and prices. Any online retailer or LFS.
Courtesy * Integrity * Perseverance * Indominable Spirit * and Self Control
Tenets of TKD