Assuming you mean the gap between the bath & the underside of the first tile leave a 2mm gap (standard tile spacer). Fill the bath with water before you silicone, leave overnight before emptying the bath to let the silicone cure.
Same gap; usually based on 2mm tile spacers. In a wet environment (baths/shower cubicle), the corner joints where the walls meet are best siliconed rather than grouted as differential movement between the two walls can cause grout to crack.
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