Urza's Legacy is a small set with 55 commons, 44 uncommons, and 44 rares. This is the normal size for a small set of this era. Foil cards are included in booster packs with advertised odds of 1 : 100 cards. This is the first set where foil cards are included in booster packs. The set was printed in the USA and Belgium. 
Urza's Legacy was sold in 15 card booster packs. Booster boxes have 36 packs.
The US printing uses sequential collation with 10 × 11 sheets. Packs are back-facing with commons, then uncommons, and the rare last. If a foil card appears, it will appear in the last slot for a card of the same rarity in the pack. This means, for example, a foil rare will replace the regular rare. (It's not clear if multiple foils can appear in one pack; I haven't seen it.)
|11 Commons||3 Uncommons||1 Rare|
There are two common runs, A and B. Run A contains 30 cards each twice, and run B contains 25 cards each twice. The first six commons are from A and the last five are from B. If there is a foil common, it can displace a card from either sequence. (Presumably, a card from A is displaced 6/11 of the time to ensure that A and B cards remain balanced.)
Run A contains 30 cards each twice. Cards are in a repeating color pattern of white, black, green, blue, red. The choice of first card is mostly arbitrary.
Run B contains 25 cards each twice. Cards are in a repeating color pattern of white, black, green, blue, red. The choice of first card is mostly arbitrary.
 The "Compilation of Manufacture Information for MTG Products" thread on the Magic Librarities Forum confirms the existence of Belgian-printed Urza's Legacy, but I couldn't identify any Belgian openings during my research.