Basic Points

• The width of the spread is a key mechanic when designing an options strategy to make a trade. 
• In a vertical spread (both options are on the same date), the width is the difference in strikes (the target prices an option is based on). 
• For a horizontal spread (both options are on the same strike but a different date). The width of the spread on horizontal spreads is the time width, which is the DTE (DTE = number of days in between both sides of the spread). 

Intermediate: Mechanics

On a debit spread, a narrower width will usually shorten the goalpost to breakeven, meaning that a smaller move in the underlying security is needed for a profit. Narrower spreads are also generally much more resilient against premium lost from IV crush or time running out. The price debit spread holders pay for not having a wider spread is to have a lower max profit. 

Strategically, such as designing option strategies to play support/resistance on key levels, it would be more risk averse (and also to be aiming for smaller moves in the underlying) if going long with a debit that has a narrower width. On the other hand, widening the spread would make it more expensive but also increase the potential reward.

Advanced: Strategy

One type of option trade where the width of the spread is critical are iron condors and iron butterflies. For those seeking debit iron condors or butterflies, the wingtips will make the trade more expensive, but have more convexity and a higher max profit. If holding a debit iron condor until expiration, then what needs to happen for a profit is to move beyond the strike price by at least as many points as the options premium. 

Looking at this from the counterparty perspective, and writing a credit iron butterfly, then having a greater width increases the size of the overall target range for expiration. However, there is less padding from a smaller premium coming from smaller deltas, and so if there is a severe move then it is going to hurt more writing on a wider width.