1997 boxster 2.5 wheel question

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To your question:
The 16" wheels are 6" & 7" and yes a full size wheel and tire will fit in the front trunk if you lean it back.

Recommendation from an experienced owner:
Save the full sized spare idea for your Range Rover. It will consume the entire front trunk and put unnecessary weight on the nose of a fine German sports car. In 7 years of ownership and 40k miles I have never needed a spare at all. Mine is now removed and replaced with a small lightweight roadside tire flat-fix and 12v compressor.