Using leftover Romex in conduit (surface mount application in an attached garage)
You need big conduit

A flat cable like Romex is treated the same as a circular wire of the large dimension.  


With 1 cable in round conduit, the conduit inside diameter must be at least 138% of the widest cable width. 
With two same cables in round conduit, the conduit ID must be 259% of the widest cable width.
with three same cables in round conduit, the conduit ID must br 274% of the widest cable width.  


Note that at minimum sizes, it will be a bugbear to pull.  You may be wise to upsize further for ease of pulling.  

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If you are using non-round conduit, then you need to look up the cross sectional area of the conduit and compute the cross section of the (rounded out) cable.  Compute 


  cross section of cable  = (cable widest width ^ 2) * 3.14159 / 4


Cable cross section can't exceed 53% of cable cross section for 1 wire in conduit; 30% for 2 cables in conduit, and 40% for 3 or more.