"Always the same position size" wont grow your account exponentially which is what most everyone would like to achieve
And you are right because "you biggest loss is always ahead of you"
Whatever your plan is for your biggest loss value make sure to add some type of buffer for slippage
When you mention Fixed I take that as trading X number of shares per Y equity dollars in the account like 1000 shares per $50 000 in account... then 2000 when you get to $100 000 in account
The better way to position size is based on % of account... like risking 2% of the account and finding...
On a credit spread the max reward (profit) is the credit you received when you sold your spread
in your case 1$
On the credit spread the max risk (loss) is the width of the spread less the credit you received
in your case (5-1)=4$
However, this isn't your odds.... this is just your max risk vs...